You don’t. It’s not your decision to make. If the cost of repairs is more than __% of the vehicle’s current value, it’s a total loss. If the repairs cost less, it gets fixed. I left the percent number blank because it varies by jurisdiction, and you didn’t say where you live.
The decision is made by legislation, not the body shop or the insurance company, and exceptions aren’t allowed. If the repair cost goes over that magic percentage, your vehicle will be branded as salvage. That’s the law.
If the vehicle is deemed a total loss, your insurance company cuts you a cheque for the actual cash (depreciated retail) value, minus your deductible, and they keep what’s left of the car. You can buy it back from them, but you won’t be able to drive it unless you get it repaired and re-certified. Trust me, you don’t want to do that.