You clearly did not read the documentation.
they have a different template engine than webforms, and different controls. they have mostly a one of one control to the webforms controls. By definition their controls are Ajax enabled via knockout.js. you would build composite controls rather than an ascx.
binding is different than webforms. in webforms, the server builds a control tree. at render the tree is userid to create html. on post back data is bound to the control tree.
DotVVM uses the more traditional view model, and uses knockoff.js binding and html generation, that is the full render is done client side, rather than server side.
Their migration approach is to add DotVVM support to your old webform project and start converting. Once the site is fully converted, you can migrate to their .net core platform.
Microsofts migration path for webforms is:
1) migrate to MVC on 4.8 platform, then migrate to asp.net core.
2) convert all webforms to razor pages on asp.net core
3) convert all webforms to blazor pages on asp.net core
your migration will depend on how well you abstracted you architecture. Is it multi-tier and you only need to migrate the UI. Is it C# or VB.net? asp.net core really only supports C# (library projects can be vb.net)