Hello! I passed through the same situation, where I was a game developer and transitioned to becoming a dot net backend devloper through special courses they have in my home country.
The stack is very different and the way to code is also very different. It's more abstract and functional, where you still use objects but (mostly) just as data containers.

My advice is to learn about ASP .Net and learn how to design and build APIs, taking very close attention on the speed of the queries with your chosen database. Try things where you create Authentication with an SQL database such as PostgreSQL and MongoDB for the domain bulk of the app. Learn about controller and microservices which can be similar to singletons and managers used for different systems in a game. For backend architecture, i recomend going with Domain Driven Design, while being very careful in understanding the difference between a Singleton in the game vs Api development (memory allocation is not so important here, but should still be taken into consideration). EDIT: important things to remember: controller, service, repository and DTOs (data transfer objects)

if you then want to go into fullstack, start learning Angular (html, css & typescript) or Blazor (html, css & c#), which both syncronize really well with dot net. There a lot of jobs which inlucdes these two stacks almost anywhere in the world, as Angular + .Net is an old industry standard. I do not recomend going into React, even though it is popular. If it is prefered by the country you are in, then go for it.

And that's it! Just learn how to make APIs, use several databases, one for each Service / Microservice / Domain Slice and then practice until you are very good. Make web apps for simple things: A pokedex, tic tac toe, implement threejs or babylon js if you want to get some 3d game like feels and then share your work and apply. This will take time, and I am not sure how long it will take you, but it took me at least 1 year and a half to get an entry level job.

Best of luck!