How to grade a route when cumulating grades for multiple sections? Is it based on the overall feeling of difficulty or is there some cumulative formula to apply which yields a more or less accurate grade?

In the following explanation, I'll use the french sport climbing grading system (see link for details).

This question is better explained by using an example:

The route "Chilam Balam" is a long route (80m) which is made of 3 sections. In https://www.youtube.com/watch?v=t-0uyFWeOoE (french), the climber says the first section is 8c+, then 8c, then it's not clear but let's say 8b.

This route was rated 9b+ during the original ascent, but then 9b by Adam Ondra and even 9a+ by other repeaters.

Is there a formula where 8c+ + 8c + 8b = 9b?

In bouldering too, I heard things like "it's an 8B that goes into an 8A+, so the overall grade is 8B+".

If there is no formula, what is the difference between a short 9a route and a long one? Like a long 9a is just an 8c that is really long?

I also heard that you could explain sport climbing routes using bouldering grades, for example a route that contains a 7A boulder problem is ~8a (that is, you add +1 to the boulder grade to get the sport climbing grade).