How to attach cms to a static site?

Question

htaccess

There is a static single page website written in html. Now there is a need to tie an online store to it, only a catalog of goods. It is planned to use some popular cms for this. How to do this so that the static remains static, and cms is connected only when you follow the link to the directory. I don't really like the option with a subdomain, because I heard that it is worse in terms of SEO.