“Substitutes” is probably not a useful word here, because you can’t just plug them in, Webflow would be exporting the HTML design and artwork for your site, and then the rest of the programming to turn that into a system is all you.
Form posts is pretty straightforward, just Google “forms processor”, there are a lot of them, and depends on how you want your forms to work and what you want them to do.
The CMS is a much more complex part. Depends entirely on your needs, perhaps start exploring AirTable and headless CMS’s to begin understanding capabilities and options.