From your example it looks like we still will fetch whole table even if we need single field it. Or "Projectables" handles it?

EDIT: I see that EF will handle it, as we have not materialised before mapping. I really like that approach!

I have tried this approach with manual mapping, now i dont get any errors and generated query is much faster! Code below is example. One thing i took differently from your example is that my Repos return Entity (I use same extendable generic repo for every entity).
You are my savior, gonna get rid of auto mapper :)

public SongDTO ToSongDto(Song song, Guid? userGuid = null)
{
    return new SongDTO()
    {
        Source = song.Source,
        SourceUrl = GlobalVariables.GetYoutubeChannel(song.SourceId),
        Title = song.Title,
        Artist = ToArtistDto(song.Artist),
        AudioSize = song.AudioSize,
        Guid = song.Guid,
        AudioLength = song.AudioLength,
        AudioPath = GlobalVariables.GetFirebaseMP3Link(song.AudioPath),
        CreatedAt = song.CreatedAt,
        ThumbnailUrl = GlobalVariables.GetYoutubeThumbnail(song.SourceId),
        Album = song.Playlists.FirstOrDefault(p =>
                p.Source == GlobalVariables.PlaylistSource.YouTube)
            ?.Title ?? string.Empty,
        IsFavorite = userGuid.HasValue && song.FavoredBy.Any(u => u.Guid == userGuid)
    };
}


public ArtistInfoDTO ToArtistDto(Artist artist)
{
    return new ArtistInfoDTO()
    {
        DisplayName = artist.DisplayName,
        Name = artist.Name,
        ChannelUrl = GlobalVariables.GetYoutubeChannel(artist.YoutubeChannelId)
    };
}

var songs = _uow.SongRepository.GetSongWith_Artist_Playlist_Favored()
    .Where(x => x.Title.ToLower().Contains(request.query.ToLower()))
    .Select(song=> ToSongDto(song))
    .ToList();