How to set up a backend for a ready-made website?

Can you tell me how to integrate strapi into a ready-made project? my workflow does not allow you to set up a backend from the beginning and then create a website-get collections and style them…my workflow looks something like this: design layout using webflow, quarkly, plasmic tools as a result, I have a ready-made interface with all collections and interactions how do I now connect strapi and reflect all the logic created in my interface in strapi???