Generally, ending concentration ends the spell (i.e. its duration ends). Web, particularly, only lasts as long as its duration. There is a possibility that a spell might not be written this way, but most concentration spells are: For the duration, EFFECT happens. If the duration ends (either time runs out or concentration ends) then the effect stops.
Just a quick one in relation to Web...
The spell requires concentration...
So if my PC casts web on a wall, and then on their next turn casts another spell requiring Concentration, does the web disappear or fall?
Yes. Cast another concentration spell the web vanishes. Sometimes you can use this on purpose, but usually it is a major limit on spells.
Generally, ending concentration ends the spell (i.e. its duration ends). Web, particularly, only lasts as long as its duration. There is a possibility that a spell might not be written this way, but most concentration spells are: For the duration, EFFECT happens. If the duration ends (either time runs out or concentration ends) then the effect stops.
They disappear.