I was wondering, how does a normal training day look like for the higher-rated players? How much time do they allocate daily/weekly for opening and endgame preparation, tactical training etc? I assume that most of them know openings and endgames quite well, so what is that they try to improve, and how are they doing it?
This question is too broad, everyone is training differently. There is also much more motivation on the side of strong young IMs than on side of weak GMs. A lot of colleagues just play, rarely train something, teach children, write articles, books, makes videos and try not to go much under 2500. They made a lot of hard training to get there, but are not working much anymore. Players of these strengths not overly motivated try to learn something from the additional chess work they do and I think the majority of them suffers as they recognize that such a chess work don't help them much to get better. But it probably helps them not to go down fast. And it certainly helps them get some money as this strength players often don't have hard everyday jobs. For me, team championships are all that matters and I try to be there always as well prepared as possible. I play tournaments too, but usually I prepare here much less and sometimes I just treat them like holidays. This is quite typical for my region, different parts of the planet will view things differently.