I am new to repository pattern. If I want to join two tables, where should I implement the logic? I have implemented it as shown below. Is there any better way to achieve the same?

This is my UserRepository class:

public class UserRepository : Repository<UserMaster>, IUserRepository
{
    public UserRepository(DbContext context)
        : base(context)
    {

    }
    public UserMaster GetById(int id)
    {
        return FindBy(x => x.Userid == id).FirstOrDefault();
    }

}

This is the Service layer from which I am implementing data access functions:

 public class UserService : EntityService<UserMaster>, IUserService
{
    IUnitOfWork _unitOfWork;
    IUserRepository _userRepo;
    IRolePrivilegeMapRepository _rolePrevilegeMapRepo;
    IUserRoleMapRepository _userRoleMapRepo;

    public UserService(IUnitOfWork unitOfWork, IUserRepository userRepo, IRolePrivilegeMapRepository rolePrevilegeMapRepo,IUserRoleMapRepository userRoleMapRepo)
        : base(unitOfWork, userRepo)
    {
        _unitOfWork = unitOfWork;
        _userRepo = userRepo;
        _rolePrevilegeMapRepo = rolePrevilegeMapRepo;
        _userRoleMapRepo = userRoleMapRepo;
    }

    public List<int> GetUserPrevileges(string UserName)
    {
        var rolePrevilegeMap = _rolePrevilegeMapRepo.GetAll();
        var userRoleMap = _userRoleMapRepo.GetAll();
        var userMaster = _userRepo.GetAll();

        var Privs = (from  rpm in rolePrevilegeMap
                     join urm in userRoleMap on rpm.RoleId equals urm.Roleid
                     join um in userMaster on urm.Userid equals um.Userid
                     where um.Username.Equals(UserName) && rpm.IsDeleted == false && urm.IsDeleted == false
                     select rpm.PrivilegedId).Distinct();

        if (Privs.Any())
        {
            return Privs.ToList();
        }
        else
        {
            return null;
        }

    }

}

I have injected IRolePrivilegeMapRepository, IUserRoleMapRepository, and IUserRepository using Autofac to the UserService class for joining the tables.

    var rolePrevilegeMap = _rolePrevilegeMapRepo.GetAll();
    var userRoleMap = _userRoleMapRepo.GetAll();
    var userMaster = _userRepo.GetAll();

The GetAll method is of return type IQueryable. After that, using that result, I have done the join. Is this the right way to approach? Or Is there any better way to implement the same?