When I first started studying RSA, I found that I need to know Euler's theorem, to understand that I need Fermat's theorem, and to understand that, I need the order theorem of Lagrange which I am trying but don't understand as I never had mathematics as a subject. Everything I did is from my own.
I want to understand more visually rather than just maths.
I reckon from other posts, that to understand the relation of $N$ in RSA which is the composite prime of $pq$, with the Totient of $N$, I need to understand group and order. Which I don't. Other problems of RSA are for crypto.exchange. But this part of maths I want to understand first.
If you would be kind enough to explain, I would be so glad. Thanks :)