How to study chess in lockdowns?

In March 2020 as the world was heading for a lockdown, so was the chess world. Apart from the frustration that playing over the board was no longer possible, I saw it as an opportunity to work on my chess game and to come back very strong when normal life would resume. However, my good intentions did not really materialize and when I was able to play over the board again, more than 17 months later, I was by no means the strong player I would have liked to be. Now that Europe is in lockdown again and the possibility to play chess is seriously limited, it is time to do better. How can you stay in shape during lockdowns?