I worked for a dealership that did a fair bit if work for the insurance industry. The bike would get towed to us for the estimate. A mechanic would go over the bike with a fine toothed comb to make up the parts list and a labour time estimate. How fine might depend on whether the owner really wanted the bike back or not. A parts guy would sit down with the parts fiche and make up the parts estimate using all new OEM parts. I would then put these together into a proper quote for the insurance company.
An independent sucontracted appraiser would come, look at the bike, take some photos, talk to a salesman about the bike's value and use the quote to make up his report to the insurance company. If the quote was within something like 80% of the bike's value, it would be considered a write-off.
Sometimes there was a little 3-way haggling between the owner, the insurance company and us as regards parts that could be repainted/repaired/ignored vs replaced, or that could be replaced with used/aftermarket vs new OEM, to bring the repair cost down if the owner really wanted the bike repaired, or we really wanted the work at that point.
If the bike was written off, a wrecker that had a contract with that particular insurance company came and picked it up, and payed a bill that included labour fees for the estimate, towing fees to get it to us, and a per-day storage fee. It wasn't uncommon to see that bike sitting for sale as roadworthy out front of the wrecker's afterwards, haveing been repaired with used parts, bondo, paint, welding etc, which would would not have cost the wrecker much at all.