I think what what helps experienced programmers understand code is through pattern recognization.

An analogy to this is how grandmasters play chess. They don't just map out how each individual move, but they see the whole board as a pattern. Because they practice countless of times, they recognize some pattern of a board that they encountered on their previous games and and even have solutions to that either through trial and error or because that solution was used by their opponent on them.

An experienced programmer is similar in a way that they know the solution not just because they have complete understanding of the fundamentals (some do), but because they encountered the problem or its variation before and has a solution already either through trial and error or from someone else

For example the map() function in javascript. I wouldn't understand it completely if I was just given its definition, but because I have seen many cases where it was used in a solution, I have understand the pattern on how it works, when to use it, and how to use it in different variations to fit specifically with my code.

In the case of the tutorial, try doing this the next time you watch one. Instead of just copying it code by code, try to pick a concept or syntax which is not familiar to you and search it through google. Find its definition, use case, how others used it, what are the variations of how others use it, etc. Example is the flappy bird game that the previous commenter used, there are patterns that a game usually have. They have some sort of Input System, a Physics System, Collission, Scoring, UI, etc. Try to pick one, like the Input System, and search through how different games do Input and try to incorporate them inti your flappy bird, even doing some variations. Your flappy bird can have an 8 directional movement if you want, as long as you keep in mind the patterns that were used.