https://xmpp.org/software/ has a wealth of information about server and client SW. Here is a list of providers for a free account: https://providers.xmpp.net/.

I run my own XMPP server using metronome, but I also run a Simplex server I prefer to use.

On top of the links shared - I would recommend Snikket (https://snikket.org/) if you're getting started.

They provide a server you can self host, and apps for Android and iOS. Everything is designed to work together and offer a good onboarding experience.