I suggest that you first figure out how to host the ASP.NET Core 3.1 solution on local IIS on your development machine, publish it to local IIS and execute it on local IIS.
Then you need to find a Web hosting provider and ask tech support on how to create a ASP.NET Core publishing package that will be hosted on its hosting site.
Note: If you are using MS SQL Server Express LocalDB, then you will need to ask about how to deploy the MDF file to their Web hosting database and stop using LocalDB. This will hold true to any database you used locally. How to get your locally used successfully deployed to the Web hosting's database.