What does yeast do to flour?

Does yeast overtake flour?

That depends on what you mean by "overtake". The most obvious interpretation is that you are asking if you wind up with more yeast than flour. If so, then the answer is no, not even close.

if yes then does it become protein from carbs?

Yeast begins its process by using enzymes to break down the complex carbohydrates that are in the flour (starches), into simpler sugars. They then eat the sugar, producing carbon dioxide in the process which causes the bread to expand (rise).

The process can be accelerated by adding some simple sugar (e.g. table sugar, a.k.a. sucrose) so the yeast has something to eat before it has broken down the starches.

As part of its biological process, the yeast does have to build protein molecules, so technically, yes. New proteins are added during the process of the yeast consuming the starches in the flour.

When we start with 100% flour does it become 70% flour 30% yeast germs after fully fermented dough?

No. Almost all of the flour remains, even after extended periods of time.

The use of yeast in bread-making is highly varied. Some breads use very little and rise for short periods of time. Other breads use more, and/or involve multiple rises, during which a relatively large number of yeast cells are able to reproduce.

As a very rough estimate, one might use a 0.5 to 1.0 grams of yeast for 500 grams of flour. (It doesn't need to be precise; if you start with less yeast, you can just let the dough rise for a longer time…the yeast you put in makes more yeast!)

In ideal conditions, yeast can double in population in about 90 minutes. A short rise for bread might be 45 minutes, increasing yeast population by about 40% at most. Other breads might rise long enough for the population to double or even triple. These are optimistic numbers; very long "rises" often involve cooler temperatures, and even a normal rise generally isn't actually going to produce the maximum reproduction rate.

In the end, you might wind up with at most two or three grams of yeast for 500 grams of flour, a very much not-significant amount compared to the total weight of the bread (which also at the very least includes added water, and sometimes eggs, oils, or other ingredients). At the most extreme, you might start with 0.1% yeast by weight and end up with with 0.6% yeast by weight, with practically all of the flour you started with.

The amount of added protein is negligible, and likewise the amount of reduced carbohydrates.