For The GTAV MiG-21
Here https://www.gta5-mods.com/vehicles/mig-21-bis-fishbed-add-on-vehfuncs-v

Once installed with their instructions, open the dlc.rpf with OpenIV. I created two additional .ytd files as putting them in the single provided .ytd file caused it to become corrupt.

To access the additional .ytd files, you will need to overwrite the vehicles.meta (...\GTAV\mods\update\x64\dlcpacks\mig21b\dlc.rpf\common\data\levels\gta5) file with the one in the zip file. These lines at the bottom of the vehicle.meta file allow additonal .ytd files.

vehicles_jet_interior
mig21b1

mig21b1
mig21b2

mig21b2
mig21b

Follow this path to add the provided .ytd files:

...\GTAV\mods\update\x64\dlcpacks\mig21b\dlc.rpf\x64\levels\gta5\vehicles\vehicles.rpf\

Toggle "Edit mode", top right button, and drag mig21b1.ytd and mig21b2.ytd from the extracted zip file and drop here:

...\GTAV\mods\update\x64\dlcpacks\mig21b\dlc.rpf\x64\levels\gta5\vehicles\vehicles.rpf\mig21b1.ytd and .../mig21b2

Just to confirm the livery, I'd click on the mig21b.yft file to open the OpenIV Model Viewer to see what the model will look like with the new livery.

Once satisfied, close the model viewer and run GTAV and spawn mig21b with the trainer of your choice.

I've included samples of my work files, in case you'd like to use them as a starting point for your own work.

Enjoy,
Gillman