I am starting a project with prosgresql, EF Core and .net Core. For this case is it a must to use npgsql?
It isnt a must to use npgsql you can use EF Core on its own https://www.npgsql.org/efcore/index.html.
However I would recommend using npgsql apposed to EFCORE, I use it for when working with .NET CORE and PostgresSQL they also have great documentation at https://www.npgsql.org/doc/basic-usage.html and you don't need to depend on EF