Restaurants prep potatoes ahead by cooking the potatoes partially.
The partial cooking of the potatoes when restaurants prep them ahead can be par boiling, roasting or steaming the potatoes, which reduces the final cooking time of the potatoes significantly.
After the restaurants prep their potatoes, the potatoes are then often held in ovens set at a low temperature or even warming drawers.
For some potato preparations, like for mashed potatoes, they are made ahead and then finished with some hot cream, just before they are served.
And for raw cut potatoes, the restaurants store the raw cut potatoes in submerged cold water to prevent the potatoes from oxidation and browning.
Restaurants will often peel, boil and mash the potatoes ahead of time.
Then just before serving, the mashed potato mixture is then reheated by mixing it with some boiling cream or milk to achieve a creamy texture.
The best oil to cook potatoes in is canola oil, vegetable oil or avocado oil which are high smoke point oils.
High smoke point oils are best for cooking potatoes in, as the high smoke point oils also allow for crispy results, while also not affecting the taste of the cooked potatoes.
If you want a richer flavor to your cooked potatoes, you can use lard, bacon grease or even duck fat or goose fat.
Other great options for cooking potatoes are ghee or even a mix of butter and a high smoke point oil, which can help to prevent burning while also still getting some butter flavor.
I prefer using canola oil myself when cooking potatoes.
Peanut oil is also another great cooking oil for cooking potatoes in.
Some restaurants also use peanut oil for cooking their potatoes fries in.