I think it's important to note that the insurance companies don't quite estimate the damage. They send the car to a workshop and let the car workshop estimate how much it will cost to replace parts. Then they'll calculate your excess and all that and tell you how much they will let you claim.
Here's a pretty good link:
And just a comment---state laws have rules about when a car is "totaled." That's why there is a category called "salvage" titles.
Insurance companies estimates car damage by determining the value of car and by cost repair. You can also get free auto repair estimate online Winnipeg from experts.