How to let global users registration from the website frontend?

Hey there. I’d like to create smth like a SaaS WP website builder. How to create a registration form for new users, which will then be able to create their own websites (subsites of Multisite or even networks) under subdomains or domain/names or own domains (all those settings also need to be inside their WP admin. Maybe some WL WP plugin to also simplify admin? Any hosting like Digitalocean or another that best feed for only several not critical websites so far? thank you!