TL;DR: EAG affected how I see EA and my activity in it.

I'll write a drafty version here based on your questions:

How you prepared for EA Global: London 2021

Mainly: I wrote a post inviting people to talk to me about a very specific topic (software careers).

I also wrote a swapcard profile inviting specific conversations, mainly about side projects of mine, inviting people to either comment on them or use the services that I'm offering in each project. I also thought of this as checking product-market-fit: Do people actually want what I think they want?

What you did and found most valuable during the conference

1. I had 8 conversations with potential mentees.
2. I had a meeting with some "high ranking" EAs, and following at least one of those conversations (which I won't elaborate on without permission), it clicked for me that "the people running EA are only humans" and that "I can do it too". I don't expect what I wrote here to resonate with anyone, we've all read it before, and that's exactly the point: There was something about actually seeing it, actually hearing it, that I don't think I could by reading an article.
3. I met a friend who I only know from video calls. I discovered he's really tall.

How you followed up with people

I sent everyone relevant my calendly+Telegram. 

How all of this went for you

1. Keeping conversations open is working well.
2. The first mentoring conversations were valuable because I was really worried about culture gaps, and face-to-face is high-bandwidth. Note I am comparing them here to starting with video calls
3. The focus on "I must post this before EAG" forced me to get the post out already, which "got me out of my shell".
4. That was my first post, and since then I've been reasonably active here in the forum
5. Which has also led to people talking to me about EA and Software, which is.. exactly what I want!!
   1. I really want to get to know people in the global community, but I also think that posting something like "I'd be happy to talk about anything!" would have been too generic and wouldn't work. (It would also be less fun for me than talking about what I'm most interested in)
   2. This might lead to something more concrete (like working with people on software projects), though - who knows
6. Some of the conversations were really fun. For example, with one founder, we walked around and did user testing with people in the conference. (Did two people ask you about a travel related startup? Those were us!). This also would not have happened through video.

Whether you wish you had done anything differently

After 2 days of the conference, with only one more day to go, I understood I am focusing almost entirely on "helping others" with almost zero "helping myself". Until then, I told myself that this WAS helping me: It was helping me discover if some of my side projects had a future.

At the evening of the second day I told myself "I am just intimidated by the 'important' people here", this is a known psychological failure mode of mine, and it's better to just reach out anyway and let them filter me if they want to. So I sent out many messages to 'important' people that I wanted to talk to, especially to get career advice for myself. Almost none of them wanted to talk to me (that's ok!), but one did, and that talk made a big difference (see point 2 at "What you did and found most valuable during the conference").

I wish I have reached out earlier.

Adding: I also wish I had more quality time with people from my local community, but we were all SO EXAUSTED. Wow, nobody talks about how exhausting it is to have a whole day of 1-on-1s