I hear you about local dealer's parts department being less than helpful. They should be able to show you an exploded parts diagram covering ANY part on your 4Runner.
Two suggestions.
First, go to the official Toyota parts web site at:
Toyota Parts and Accessories - Official Online Store. There you can see the exploded parts diagrams to figure out what you need and the part numbers for the various parts.
Second, get the official Toyota repair manual at:
TOYOTA 2016-2019 4Runner Repair Manual RM27F0U. This will show how all the parts fit together and any special procedures needed for disassembly or reassembly.