I have been searching on EF Core documentation, if adding .HasIndex() on your entities mappings would bring any benefits on a DbFirst scenario, and I couldn`t find anything.
I have this 20yo DB that has all the necessary tables and indexes created, and I am mapping some tables to query them using EF Core. I wonder, what could be the benefits of mapping the indexes on a DbFirst scenario where you would never update the tables schema via code? Does it affect the way EF generates the SQL queries?
None. HasIndex would only apply to creating indexes for code-first/migrations. You don't need to map indexes for EF to generate or optimize the query.
I do recommend after introducing EF to a project to record/report on the most common queries executed to determine whether there are new indexes or adjustments to existing indexes that might benefit your application's performance. (I.e. included columns)