Is Hive the new place where all the old guys migrated to?
Yes. Hive was launched and supported by a distributed team that were the old witnesses and community developers on Steem. All the SteemitInc Developers also resigned soon after the Tron acquisition. So very little left there of what you would have seen in 2016-2019.

What is the difference between Hive and Peakd?
Peakd is just a front end built and operated by a different team than the community developers contributing to hive.blog. I personally find peakd better to use, but it's a preference thing.

Are there more platforms like Hive and Peakd?
Hive account@ecency is probably the closest alternative. My team have also built a WordPress plugin Hive account@exxp (formerly Hive account@steempress) that allow any WordPress blog/website to integrate Hive as a way to share their posts here and also allow visitors to log in with a Hive account to comment and upvote on the blog page.

What is the suggested platform where most of the old Steemit users are writing
I guess there's an even split between peakd and hive.blog. But remember that anything you post on one will also appear on the other. As they're all utilizing what's stored on the Hive blockchain.

How does communities work and which communities are the best to join?
Communities are just like sub reddits. I suggest you browese https://peakd.com/communities and see if any of the popular communities relate to your interests.

Oh, and welcome back :)