Unfortunately ASP.NET Core doesn't support web forms (iirc), but if you have clean separation between backend and frontend, you could change frontend to mvc/razor pages/blazor.

Tim Corey has a good video about migrations: link

ohooo dude you are fucked to put it simply maybe you can try avalonia ui for frontend or asp.net core mvc but it will be hard Good luck and keep updating us if you can

One at a time essentially.

If they all communicate use something like YARP to sort your routing out.