On certain heists, you can gain and use heist specific favors in the ready up screen. For example, on Road Rage, you can place an alternate loot pickup that's active immediately. How do you gain heist specific favors?
Heist favours in PayDay 3 can be purchased or found during the quiet phase during heists.
I have found this guide on Game Rant (Find the guide under this link) regarding PayDay3 favours.
Especially this excerpt might help you:
Players need to flip to the vendors' tab in the main menu to purchase items. Favors can be found all the way at the bottom of the weapon dealer Gage's shop inventory. They will be right underneath the weapon stickers. Favors only cost $10,000 and are well worth the small investment. Check back between each heist for different items. Be on the lookout for unique items like QR codes that will make certain heists easier.