The way insurance works:
Assume a $50,000 car is damaged.
Assume $40,000 to repair.
Assume $10,000 scrap value.
So insurance can repair for $40,000 and you get your repaired car back.
Or...
Insurance company pays you $50,000 (totaling the car) and gets $10,000 selling the scrapped car.
Either way, the insurance company is out $40,000 and you are "made whole."
The adjuster does not actually use a value like 70% or 80%. The adjuster looks at car value, cost to repair, and scrap value. The adjuster decides based on the bottom line to the insurance company. But the decision one way or the other is usually around that percentage.
This is not the insurance company ripping off the car owner, it is a simple business decision.
Now with this knowledge, assume the car is worth $20,000. With $40K in repairs, that car will be totaled. No question. Even if the scrap value is $0, it is cheaper to total it.
Now assume it is a $100,000 car, still a 40K repair, and a higher $25K scrap value. This car will be repaired.
There are complicating factors such as repair backlog, parts not available, and undiscovered damage until repairs begin. But the basic math is the biggest factor in an insurance repair-or-total decision.