Today, they have huge prep in the openings, but are on their own in the endgames. Years back, there were adjournments, where a player like Karpov got tons of help (it was not the same for western players, though) and were able to play perfect endgames. Maybe that is part of the explanation why current GMs look a bit weaker in endgames (whith remarkable exceptions, like Carlsen)
The players did quite a bit of that adjournment analysis themselves, and they did check the work of their seconds before resuming play. Plus independent off-competition analysis. Having adjournments surely boosted their endgame knowledge while today's short time controls often mean that endgames are played in semi-constant time-pressure.
If you really wanted to improve more, after reading a few basic chess books ofc, spar against chess engines. Takebacks allowed ofc. After playing 50 games and losing 50 games, I guarantee you're a better player before the sparring session.