Is it a very common practice to execute a specific build order in a master league? I wonder if it's really a standard way of playing, or players just improvise efficiently without using any builds at all.
At higher levels you do not use Build Orders as you know them, like:
Instead the pro players just memorize the order they want to get. For every sequence they have sub sequences they can substitute. The key is to know the results.
For example [Supply, Barracks, Gas] and [Supply, Gas, Barracks] lead to the same buildings, but have different effects. The former allows you to get units faster and you have more Minerals, the later lets you harvest way more gas but you are very vulnerable. Another good example is Protoss: Do you create or skip the first Zealot?
With scouting and other information like not seeing Mutas at 10 minutes etc. they know how to react and change the order of their sub sequences accordingly.
Once you know how to macro properly you do not need supply or time information, as you just get the buildings as fast as you can without missing production cycles.
You do not need to be Master league for this to kick in, usually around Diamond you change your way of thinking in hard Builds and everything seems to "just work".
Still Build Orders are a good way to share Builds and even Pros need a guideline how to execute a completely new strategy they have never used before. They will look at the build and memorize the key elements.
First of all, a build order should NOT be interpreted as a step-by-step guide, mainly because it's dependant on map, opponent's actions etc.
While it's easier for a relatively new player to memorize it and learn the build order, I'm pretty sure master-level players do have perfect execution timing, but whole order of actions is much more complicated than ~15 steps.
So basically it's a yes-and-no answer.
Well overall the answer to this is going to be no. They might have a build in mind, but at master level having the ability to react to the other players decisions/timings is crucial.
Given the high level of micro skill and variance in strategies at that level, I think it would be unwise to blindly commit to a build. This works at lower tiers however at master your opponent is going to be much quicker to scout you and much more able to utilize scouting knowledge to punish you.
Being reactive is key and sticking to a build order is the opposite of that.