The original question was about the use of these two services. I answered their question and provided a link. I’m sorry that you took that as advertising, but the OP was asking for the basics. There are more productive ways of responding than immediately attacking, though, that would have helped get your point across. For example, providing links to things like Headscale or Netmaker to help your point about not using a third party service.

I’ve been using Tailscale because I like the business model they’ve been pushing. They’ve also followed through with that, expanding what the free tier of their service offers instead of restricting it more.

Although possibly biased, Tailscale has a write up comparing the two and the conclusion is that both are good at accomplishing their goals. If you have a technology or business preference, it could mostly come down to that. https://tailscale.com/compare/zerotier/