Much like sideloading during the initial install process, sideloading firmware upgrades or XDA apps can be done the same way.
Pre-requisites:
apt install fastboot
on Debian)First, connect the phone via USB
Make sure ADB is on:
adb devices
Now reboot, to recovery:
adb reboot bootloader
Once on the menu, navigate to Recovery
TWRP should load > Advanced > ADB Sideload
In your PC's CLI/terminal, reference the path where your .apk or flashable firmware zip is stored:
adb sideload /home/angela/Desktop/firmware.zip
Check the TWRP output to ensure it was a successful flash. If there are errors, they're typically pretty descriptive and will tell you if a dependency is not met.
If all is good, reboot back into the system.