Where can I find free training tools like detailed annotated chess openings, middle games and endgames in the PGN format? I have decided to study chess after playing it leisurely for a while. I looked at some online chess courses like the ones offered by Igor Smirnov at the Remote Chess Academy, but these options seem to be quite expensive ($700+). I looked at one of these courses that my friend purchased and it contained some openings that were annotated and showed variations of an opening:

I was wondering whether there are freely available resources like the one in the picture below that have detailed annotations of openings, middle games, endgames, tactics, etc. If so, where would I be able to download them?

Alternatively, is there is any free/inexpensive software that acts as a trainer of some sort where one can practice openings, middle game strategy, end games, tactics, etc?
You can have a look a chess.com forums where people share a lot of PGN games.
pgnmentor.com offers PGN sorted with openings, middle game themes and endgame themes. If you are looking for an all in one platform, chessbase is the best so far, a bit expensive but will serve you well.