GEA stands for "General Equilibrium Approach". It deals with solubility and a ton of different ions that affect it [solubility].

It doesn't deal with the solubility, it is much more powerful: it allows to calculate equilibrium state of EVERY solution, be it acid/base, solubility, complexation or any combination of these.

Actually every other approach to any equilibrium problem is an approximation based on simplifying assumptions, GEA treats every possible side reaction as if it was an important part of the system.

So, broadly speaking, the subject is just equilibrium. General case of every equilibrium calculation is always the same. System is described by set of equations: mass balances, charge balance, and equlibria for all substances present in the solution. You solve it - and you know what are equilibrium concentrations of all ions and substances. Once full set of equations is obeyed there is no need to separately think of dissociation or hydrolysis, protonation or complexation, dissolving or precipitation - all these are taken into account.

This is never trivial to calculate, which is why we go for simplifying assumptions.

Sorry, no idea about YT videos.