I’m sure laws on this differ everywhere in the world but I assume you’re talking US. It is doubtful an employer could win a law suit against you for not showing your specific methodology unless you have a contract and that was part of it.

As far as firing goes, there aren’t very many situations that an employer can’t fire you over for cause but obviously also can fire you without cause.

Would they own the templates? Yeah but they’d also have to know to look for them unless you told them. Otherwise they’d probably already have created some templates and expect you to use and perhaps improve them.