Which free Services for backend?

I have been codeing saas Websites with AI for very long, but I always have the problem that I cant really run my SaaS because I am only capable to code websites Frontend with html css and js. I now want to add Databases or interactive Server behind my Websites to actually make my SaaS s running. Which free Tools could I use to add to my existing front and Code to be able to actually run sign forms and a real interactive SaaS? Thank you!!!