This question is probably too broad for anyone to answer. "endless map" can mean a lot of different things depending on the game. For example, in something like Flappy Bird, you just randomly generate a pipe on the right and move it to the left. In a runner, it could be as simple as every second or so generating a "jump" block or a "duck" one.
As your map and terrain get more complex it can get much more involved. This is not a Godot-specific thing. There are lots of resources out there if you search for pcg techniques.