SEO on a Non-WordPress website

I have only learned and applied SEO on a WordPress website currently using the RankMath plugin, how different is it applying SEO on a website that has been built using code?

I know there will be differences such as applying schema differently but I am not sure how much of a difference there really is; I know a lot of aspects will remain the same.

Is it worth me taking on a client that has a website built using code, or will it require a steep learning curve and am I better off just focusing on clients that use WordPress as their CMS for now?