The goal in bodybuilding during a gaining phase is to be in a very slight calories surplus (200-300 calories above maintenance, at most) to maximize the amount of time you're building muscle before you need to cut again (bring calories back to a deficit to shed body fat).

Tracking scale weight is difficult because shifts in water weight and hydration can swing the scale 5+ pounds in either direction without any change in body fat. So I pair scale weight with a 7-point skin caliper measurements taken on a weekly basis, along with waist circumference, in order to infer whether body fat is trending up or down. And also take weekly progress photos of 6 angles/poses with consistent lighting, which I share with a coach.

And then you pair that with weighing and logging everything you eat, and you can make small adjustments to your meal plan on a monthly basis to try to stay in that 200-300 calorie per day surplus for as long as possible. (Although most bodybuilding coaches adjust diet based purely on how your physique is changing in weekly check-in photos without the need for measurements, but I like extra data)

> down to how many reps I did of which exercise, with how much weight)

I also do this. Track every exercise, every weight, number of reps. It's necessary for knowing whether you're progressively overloading over long periods of time. Progressive overload becomes harder to measure once you're past newbie gains because you can't increase weight every week, so some weeks the goal is just to squeeze out an extra couple of reps. Which adds up over time

This is obviously excessive for 99% of people. But I enjoy doing it as a hobby. I would absolutely not recommend this level of tracking for health reasons (not necessary) - I find enjoyment in the process.