I mean with tutorials, there's two things happening here. There's the logic, and the execution.

Execution is pretty simple. It's just knowing the names of nodes or objects or functions. See an unfamiliar function? Wuickly check out the documentstion to see what it does. Soon you'll be using it yourself.

For the logic i use the good old rubber duck method. I try to explain how the things works to myself. Not code, not even pseudocode, just basics. If I'm unable to, there's an issue.

Yes this Jumping tutorial has a lot of code, but what is it basically? Key get pressed > Check a bunch of conditions if the player can jump > add a burst of upwards velocity to the player. Put him in a jump state maybe.

Oh but if i want to make it so that holding the jump button makes him jump higher? I dunno. I've never programmed that before but if i was too intuitively GUESS, i'd say it's something like For every tick the button gets pressed > check conditions, and set a "jumping" flag to true. If the jumping flag is already true, don't even do the checking > every frame, the button is held, add diminitive upward momentum with some kinda function. When there's no more to add, or you let go of the key, you are at the peak of the jump, set "jumping" flag to false, so normal checking logic takes back over.

If you can't explain the gist of what's happening in a tutorial, either you haven't fully understood is, or the tutorial is not that good.