do AST's automatically become rolling AST's after they expire?

Tenancy Types and Management

Hello, just a couple of questions. Just starting out. Once you've issued either a 6-month or 12-month AST, do they automatically become rolling AST's once they have expired? And is it better to do this if you are happy with the tenants, rather than keep renewing with a new AST?
Also, I've read a lot where you advise issuing an s21 at the start of the AST once you've secured deposit, how would this affect a 'rolling' AST if you remain happy with them staying? Wouldn't this make tenants believe they have no choice but to leave at the end of the original AST? I wasn't sure if it was better to serve the s21 later on down the line when you want them to leave? And which s21 would be appropriate to use? Thanks for your help