What does “Suicide after choosing a player class” mean

The individual class configs only need to be used if you have a bind conflict, as it won't matter what your binds for the Spy are while playing Scout, as long as they don't impede your other controls.

Generally, it's just laziness and not wanting to avoid all the binds in the other class configs.

Scrap banking is the process of trading scrap metal for common, low-value weapons. Typically this is at the rate of two weapons per one scrap.

If someone needs a scrap banker, they are looking for scrap and have weapons. If someone is scrap banking, they have scrap for trade.

This is effective for the person with weapons because it means not having to go craft scrap with weapons (if they even have the correct slots to be able to craft scrap), and effective for the person with scrap because often some weapons they are given will be worth more than half a scrap.