Going to Germany for 77 days and can I go to UK for 2 months, and enter Germany to take my return plane?

I'm non EU national and my passport allows me to go to Germany for 90 days without a visa and 6 months to go to UK without a visa.
My plans are to learn German for 75 days (it is an intensive course) BUT because it is a expensive long haul flight, after it, I want to go to UK to stay with my friends for 2 months and travel using ryanair. As my return ticket would be from Germany, can I enter Germany just to take my return flight to my home country? (I'm afraid as the entrance is for 90 days only). Thank you very much.

Added (1). I mean, travel Europe using ryanair.

Border Agent decides how long you can stay NOT YOU. The maximum is 90 days. The border Agent can say less.
In this situation you are making a NEW entry to Germany.(Schengen zone) A second visit.
Changing planes and staying in the International Transit area would be the easiest way.
start your return flight from UK to connect with your next flight.

That you want to deliberately make it complicated is up to you.
MOST on the planet can't afford months and months of VACATION you will raise concerns trying to return after your first long stay then another vacation and then more.

Just changing planes at the Airport in Germany would be easiest. Border Agents can refuse you entry at any time. PASSPORT is identification not an entry pass.

You should check that you do not require a student visa as you say your studying on an intensive course for 75 days, assuming it's Monday to Friday. You would be ok to enter the uk as a tourist and stay up to 90 days. You say your travelling Ryanair in Europe, the thing to bear in mind is they often fly into smaller airports, so if you are transferring from a domestic/intercontinental flight to return home, you may need to transfer to another airport.

The only question that arises from your post is: shall I be allowed to enter Germany when I go back home after my trip in the UK?

The answer depends on the actual terms and conditions of your visa exemption in Germany.

A visa exemption may Indeed belong to various categories (tourist, student, business, officials, and so on). Each exemption.is always determined by its own terms and conditions such as holding a passport valid x months after expected departure date, a return ticket, enough funds to cover local expenses, and so on (in some countries, the exemption is valid only in some ports of entrance). The terms and conditions stipulate also whether the exemption is valid for single or multiple entries.

So you have to check the full terms and conditions of your visa exemption to Germany. We can't do so for you since the exemption depends on which country you are a Citizen. You have to check the terms and conditions of your visa exemption with the German embassy in your country.

At first view, your 90 day exemption in Germany is probably valid only for a single entry and it may not help you when you travel back home through Germany. At that time, il you come from the UK by plane and if you just transit by the Schengen area, you will travel under the transit rules:

However, as stated above, you may rely only on official information from the German embassy in your country.