Okay forget everything you know about table 310.15(B)(16) and learn this. If you have a piece of THHN sitting on the table in front you it is rated 90°C.
If you are planning on connecting that conductor to a device or piece of equipment rated for less than 100 amps, that piece of THHN is now sized from the 60° column.
If you are planning on connecting that conductor to a device or piece of equipment rated for 100 amps or more, that piece of THHN is now sized from the 75°C column.
Equipment and devices manufactured for less than 100 amps have terminations rated 60°C. Equipment manufactured for 100 amps have terminations rated for 75°C. You must size your conductor for the weakest link in the system.
Now say you have determined that a receptacle is going to need 20 amp wire. You size the wire from the 60°C column of table 310.15(B)(16) and find #12AWG but now those current carrying conductors are going through a conduit with 20 other current carrying conductors. You will have to derate that wire by 50% but you’re not derating the termination (you have already figured for that) now you’re just derating the conductor which (#12 THHN) is rated for 30 Amps 90°C. Well that brings it down to 15 amps so find the conductor size that 50% at 90°C will be 20 amps.