It is a shame that you broke this out of your previous question where we could see the previous information given to you, much of which was excellent.
For the benefit of those who have missed it the thread is at
https://community.ricksteves.com/travel-forum/england/question-about-railway-from-windsor-to-london
On the face of it there can be quite a degree of confusion but the best way to eat an elephant is one bite at a time.
So I, and I am sure others after I run up to bed in a minute, will attempt to reduce this to bite-sized morsels.
You have a fair bit of knowledge but some of it is a bit confused.
To go to tourist attractions from Windsor you need to use a train to get to London, and then because all the main line train stations are in an elongated circle around the tourist attractions, you need to change onto Underground trains - like subway trains in North America (I would relate it to where you live but your profile gives no hint) - and onto buses.
The buses and the trains are privatised and run under government "guidance". The Underground also has a degree of semi failed privatisation and are very tightly run by London government.
All that means a degree of confusion to the poor passenger.
All central London buses look superficially nearly the same, many double deckers, all in red, some old, some new, all with a big sign on the front and side with the number of the route and destination, and they all appear on the bus maps, and all appear on the Transport for London (abbreviated TfL) website https://www.tfl.gov.uk/ which is full of all sorts of information including lots of maps - which may be helpful to visual learning.
The Underground map is a stylized, not to scale, coloured map. It shows the general directions that the various lines take, and where they cross, but it does not show actual routes or how close various attractions are to the various stations on the surface. For example, the Underground map shows two adjacent stations on the Piccadilly Line, Covent Garden and Leicester Square, as about the same distance apart as many others. In reality it is less than 300 yards long and much quicker to walk on the surface than descend into the first station travel the 45 second journey and then trudge upstairs again. http://www.timeout.com/london/things-to-do/londons-shortest-tube-journey-1 But it is a great general direction finder, and every tube line has a different colour assigned to it which is reflected in the stations and the tube carriages themselves.
The two train companies serving Windsor from their individual stations (to their individual terminus stations in London) are run independently from each other.
I will give lots more information in the morning after breakfast including everything you need to know about ticketing ... trust me, I AM getting there.
