Europe Startup Hubs / Moving to US

I’m British with a European Passport.

I’m interested in joining a startup after a short stint at a large tech / consulting company. Obviously startup culture in the US is the place to be. My questions:

1) How can I move into the US startup scene? Afaik only large companies are willing to sponsor, and even that is rare - most of the time you have to be senior and ask for an internal transfer (I want to work for a startup ASAP after getting my ‘brand name’).

2) Which European cities are the best for startup experience / those looking to found one day? I only speak English fluently, but am learning French, albheit slowly.

thank you!!!