Oil & Gas / Midstream Primer, Resources

• Yes, a million barrels a day is a lot and indicates a large takeaway capacity for a pipeline system. You would write that as 1 Mmboe/d or 1,000 Mboe/d (each "M" is a thousand). Energy Transfer's Permian crude pipeline system has roughly the same takeaway capacity. You can see the scale when you compare it to some of the larger independent Permian players. Diamondback's last quarter production (for example) was ~225 Mboe/d. They're a Permian pure-play company with a TEV approaching $30bn, meaning 1 Mmboe/d in takeaway capacity could in theory handle all production for multiple Diamonback sized companies. 
• One thing people in oil and gas do a lot is compare things on an oil equivalent (or gas equivalent) basis. The conversion rate the industry uses is "six". Technically you can get more specific, but everyone in the industry will use six. Just use six. For example, 100 Mboe/d is equivalent to 600 Mmcfe/d. 1 Mmboe/d would convert into 6 Bcfe/d. Note that gas units have an extra "M" over similarly sizesd oil units. Generally, you would present the units in whatever commodity the asset produces the most. For example, the Permian is generally weighted towards crude oil whereas Appalachia might be 90% dry gas. If it's mostly oil+NGLs, use units in barrels. If it's mostly dry gas, use units in MCF.
• Capacity is basically just the throughput the system can handle. Generally, more capacity means a more valuable asset BUT that's also contingent on there being production from the upstream side. The Ruby Pipeline is a good example of what can happen if the production isn't there. It's a 1.5 Bcfe/d interstate pipeline (so large) that was built to move gas from the Rockies to surrounding areas. Upstream fell off there (upstream operators have better, more profitable basins to deploy capital in) so the pipeline is operating well under its capacity, so it went bankrupt.
• Companies can protect themselves from the production vs. capacity issue by contract. Minimum Volume Commitments (MVCs) basically dictate that companies either need to put a certain volume through the pipe (and pay the fee on it) or they owe the company the difference. There are also Firm Transportation (FT) agreements which are basically just operators pre-reserving space on a pipeline system. These are common with Appalachia operators (EQT, Antero, Range, CNX, SWN etc.) because they want to get their gas out to more profitable marketes. Note also that counterparty risk matters here - you can have all of the guarantees in the world but in a bankruptcy if the other side can't pay, they can't pay. During COVID companies had MVCs and FT agreements tossed out during bankruptcy because they were seen as too burdensome and would make the post-RX company non-viable. So the midstream companies got left holding the bag.
• You didn't ask but probably worth taking a moment to classify the types of pipelines and subsectors in the midstream space. They're differentiated by commodity and purpose.
  •  Crude Oil, Natural Gas, Natural Gas Liquids are the primary ones. But there are also refined products pipelines (move products after they've been to the refinery) and water pipelines (fraccing is water intensive and West Texas isn't known for it...). 
  •  Gathering & Processing (G&P), Transportation, Storage. G&P is the system of pipelines that connect to wells and feed into the big interstate Transportation pipelines. If you look at a G&P system (or really, gathering system) on a map they tend to look like spiderwebs. This is the most vulnerable part of the midstream value chain - changes in upstream production can significantly impact their profitability, especially if their MVCs have rolled off. For that reason companies that are heavy G&P tend to trade lower. Note that the "processing" piece pertains to gas and involves splitting dry gas from NGLs. From G&P, the crude or gas flows into transportation pipelines. These are large interstate pipelines. Most of the time if you hear about midstream in the news it's because environmentalists are protesting one of these interstate mega-projects. These are good assets and transport massive volumes of gas and oil from producing regions to the rest of the country's population centers. Note also that because they cross state lines, transportation assets are FERC regulated. That means that they don't have full control over their rates, and are a bit like utilities in that respect. Storage is just what it sounds like - large tanks that store gas, crude, NGLs etc. You pay a fee and you can keep your product there. 
  • LNG should probably be mentioned as another part of the value chain. Transporting natural gas across an ocean is difficult - to viably transport it you need to basically chill it into a liquid, which lets you put it on ships and send to Europe or wherever else (where it is regassified and used). This is becoming more relevant today, but even in the current market the proejcts can be hard to get off the ground. The assets that DO already exist are massive and massively profitable. 
• Brent references a market / grade of crude and is typically seen as the European marker for oil prices. The American equivalent is WTI. Those are the two markets that people will reference  90% of the time, but there are many other markets and grades all around the world. Hydrocarbons includes both natural gas and oil and anything else vaguely similar (including coal). It's a broad term. If you're digging it out of the ground and burning it for energy, it's probably a hydrocarbon. 
• Natural gas is cleaner than coal but it still produces CO2. There are also issues with straight up methane (which is mostly what natural gas is) leaking out of pipes and storage tanks, and methane is a bigger problem than CO2. Natural gas can either be used as an input into combined-cycle gas power plants to create electricity, or it can be piped around and directly used for heating. The latter is falling out of favor and plenty of states are passing laws that make it harder to expand gas home heating. 
• Mentioned this earlier but yes, "where" the capacity is is vitally important. It doesn't matter if you have a 10 Mmboe/d system if there isn't any oil flowing through it. The best systems today are offtake in the Permian  basin (oil), Haynesville (gas) and Appalachia (gas). There are other basins, but those are the three major ones that supply American energy today. 

I don't have any primers around that are updated but what i would recommend is that you read Hart Energy (best industry publication and well respected by both bankers and Corp Dev teams) and for midstream specifically, the research of Michael Blum. He's Wells Fargo's equity research analyst and is widely regarded as the best midstream research analyst. He publishes a weekly publication ("The Weekender") along with monthly publications ("Midstream Monthly") and various one-offs. Midstream teams in Houston typically send his research around along with the weekly group-wide comps spam. RBC Richardson Barr's website is also a useful and easy way to quickly judge company sizes and day to day performance for both Upstream and Midstream.

Happy to answer any follow-ups that you have.