How do I strengthen/maintain my network?

I'm an 18-year-old from the UK who has just left sixth form (high school). I am going to a low target/high semi-target university (college) to study economics in September. While I know I am still young and haven't worked in a bank, I think IB is the career path I would like to go down.

To find out more about the industry and grow my network, I have recently started to reach out (cold emailing) to alumni of the university I am going to. I have had a great response rate (likely because few students in the UK cold email compared to the US and because the university alumni network is strong) and I have been able to speak quite a few MDs, EDs and Partners at some of the top BBs and EBs. These are obviously people I want to remain in contact with and build a relationship with, however, I am unsure how to do this.

Do I just continue having Zoom and Phone calls with them and eventually invite them out for a coffee and continue with this? Or is there another way Monkeys have found work better?

Also how often should I reach out? I know how busy they are and I am probably at the bottom of their priority list, so I don't want to be an irritant who emails them constantly. Yet, on the other hand, I don't want to be forgotten.

If anyone has any advice it would be much appreciated.