domain name, the nomenclature for whatever@whatever.com is multipurpose (email/intra-instance mag addresssing)

an instance is software running on a server, being addressed at a location like whatever.com

i use amazon because its familiar, and has very mature tooling/services. it is a bit pricier... ~30%.

register a domain 'whatever.com' and then you can run any host you want lemmy.whatever.com, yourmom.whatever.com, etc.