Permanent new starters get new MacBooks, contractors get the MacBooks returned by leavers. When the devices are 3+ years old they're used as spares. Beyond that they're either sold to staff, recycled or given to charity.
at least where I live (Sweden) I know all hardware nowadays is bought in bulk by companies that eventually refurbish and resell them in poorer countries.
When I was at a BigCo, some groups would regularly send out big Excel spreadsheets of all the old stuff they were about to sell to resellers. You could buy it at the price the reseller would have paid.
Never any really good deals, just decent deals on mostly lower end hardware.
I work at a smaller remote company. We just sell them back to Apple for whatever gift card they offer. Not the highest price we can get but very low hassle.