What is a charcoal canister and a canister purge solenoid and how does EVAP work?

EVAP is short for Evaporative Emissions AKA fuel vapor. The system has been in place for many years, starting in the mid 80's. The canister is filled with activated charcoal. It is designed to capture fuel vapor and keep it from getting into the air. It does that by having a vent line that goes to the canister, then a line from the canister to the purge valve at the engine. On the other side of the system is the tank vent and its valve. So, you park the car, fill it with fuel, the purge solenoid is supposed to be closed, the vent solenoid is open. The fuel vapor from filling the tank tries to get out the vent, but it has to get through the charcoal. It gets trapped in the canister. Now you start the engine and once certain conditions are met the ECU opens the purge valve and allows the engines vacuum to pull the fuel vapor out of the canister.
During testing the system does a few things, one the fuel level needs to be correct, the engine must be warm and a few other things. Then the ECU closes the vent solenoid, and opens the purge solenoid and starts counting. It has a table in it that says for X fuel level it should take Y amount of time for the vacuum to reach the correct level in the tank. If it takes too long, you get a code. Then it closes the purge valve and counts again.it consults another table that says at this fuel level and temperature the EVAP system should leak down no more than a set amount. Faster than that and it sets a leak code. There are a few different systems used but they all achieve the same thing.
The common failures are the purge valve sticking open, that means that you fill the tank up and the engine won't start because the fuel vapor went right into the engine and now it is flooded with excess fuel just like a stuck closed choke would act.
Another failure is the vent valve sticks closed, that results in not being able to easily fill the tank as the vapor cannot get out and the stations pump will constantly shut off as the fuel backs up in the fill neck on the car.
The most common failures though are leaks that set an EVAP code. Won't really do much unless the failure is one of the two above. You just get the CEL on.
For more into -
https://www.2carpros.com/articles/how-emission-control-systems-work