How does MongoDb control the processor?

Question

I am a little confused about how MongoDb controls the processor. At the moment, the total processor is 20%, but I check that one single processor is always 100%. Check out the MongoDb user interface. I see something like this:

There are many collections in this database (~ 30), and each collection has several indexes (avg = 4 per collection). I have a client that requests a single index for a long time (and the index is always an iteration of several records), and this is the only explanation I can think of (although I read that mongo can read in multiple threads - I only perform read operations) . In parallel, I have many other clients requesting other indexes (returning only one document per request), but I do not know why MongoDb behaves this way. Does anyone know how processors are controlled? By index? By collection? What does read percentage mean? I can’t find anything.

Thank!