Doing web dev in .NET nowadays for me is mostly creating HTTP/JSON/REST APIs and using whatever FE framework you like to interface it (for me that is React or NextJS). The keyword you want to search for is "ASP.NET WebApi" or - more modern - "ASP.NET minimal API".
You could still do .NET MVC server-side rendering using "Razor" (the markup language in ASP.NET MVC - search for "ASP.NET MVC Razor".
In dotnet, it seems everyone uses the Microsoft "blessed" framework, until Microsoft does a full rewrite or loses interest - then everyone has to rewrite their stuff too.
There's no way Microsoft are the only ones capable of producing good library/framework ideas - so what gives?
ASP.NET Core is strong and overall better option when compared to FastAPI, RoR, Spring or Node.js with Express/Nest.js. EF Core with LINQ and safe SQL interpolation is one of if not the best ORMs that exist on the market across all programming languages.
This results in much weaker demand for alternatives, and combined with community that historically erred on the side of "out of box" choices, it's no surprise that there aren't really big viable alternatives to ASP.NET Core. There are for EF Core though - Dapper/DapperAOT and Linq2Db. Aside from that, most community libraries, both C# and F#, instead usually opt to build on top of them, like Fast-Endpoints or Giraffe/Oxpecker.
.NET could use a framework that is what Vert.X is to Spring, but even existing landscape is so much better than almost every other alternative, that it's difficult to complain, except, I suppose, when it comes to Identity and Authn/Authz in general that are more complex than ideal.
p.s.: I'm not saying that all "first-party" libraries are good. ASP.NET Core and EF Core, especially their "core" functionality are great. The surrounding selection of packages however can be hit or miss, and it's where a productive choice is to look for community libraries instead. It's an area that is slowly getting better.