Here's a couple of tips to get you started on this:
Scrapulances give you 500 scrap once returnong to a stronghold, there are 2 camps I know of that have them at the entrance. Otherwise, they appear randomly on the road and even less frequently once all strongholds are at 0 threat.
You will receive 740 scrap periodically once you clear all camps.
You can also farm scrap by doing death runs if you have cleanup crew project done for the stronghold surrounding it. Just thunderpoon all the vehicles then when done get out your vehicle, race will fail, then retry. You won't get Mich scrap though (30-40)
I first started getting 5,000 scrap first then using it all to upgrade all needed stats for max and Magnus opus.
You will want to allocate wisely based on the first two territories(Gutgash and Jeet) so you don't need to buy everything just buy what you need.
Here is how you will get 500-600 every 30 minutes after finishing.
Cleanup crew and scrap collection is the first two projects you will want to build in Jeet and Gutgash strongholds, this will provide you with scrap for every enemy or scarecrow you take out in these territories instead of getting out of your vehicle to pick them up.
Upgrade your Griffa ability 'Volition' as much as you can this will greatly increase your scrap intake every time you pickup scrap, fully maxed you can get 10-15 per some pickups.
Now, start to dismantle every Scotus camp that you see in these territories. These camps will be your primary source of major income once you dismantle them.
Once you do all this you can progress regularly throughout the game and get 500-600 scrap every 25-30 mins plus all the scrap at every scavenging location.
One thing I rarely utilized was returning enemy vehichles to strongholds for scraps so I'm not sure how much you will be compensated for that.
Every scrapubulance that you see attack.
Every storm you are in the middle try to get as much muthaloot as possible, there's up to 300 scrap in each crate(50 plus another 50 if your volition is maxed out.
Look for locations with scrap pallets, they can give you up to 50 scrap per pallet.
Every enemy you kill loot them if possible.
Any other tips feel free to comment and I'll add.