This guide will show you how to easily solve the garbage problems that you’re currently encountering in Cities: Skylines 2.
Make a district
Use the tool to make a district for the area you want you’re garbage processing plant to collect garbage from.
Assign Your Garbage Processing Plant
Now assign the garbage processing plant you want to collect the garbage to the district you just made.
Now the garbage processing plant won’t import garbage from your neighboring cities.
P.S. Any garbage processing plant that isn’t assigned to a district will import garbage till they can’t store anymore and any unassigned garbage plant in the district will shuffle their stuff to other garbage plants when they fill up.