How do you roast a tri tip overnight?

You need to drop the temperature more for overnight. The 'danger zone' of food is between 5°C (40°F) and 60°C (140°F), you want to limit the time your tri-tip spends at that temperature range as that's where you get bacterial growth. Anything higher than that is safe from a bacteria perspective. So you could roast your tri-tip at 145°F or thereabouts (a little extra for safety) for as long as you want.

My big concern with tri-tip is drying it out, even with the temperature dropped you can't leave it in forever without it turning into a brick. I'd be tempted to wrap it in foil to seal the juices in, then try an 8 hour roast at 145°F. What the restaurant is probably doing is roasting overnight, then refrigerating the meat for the next day's service, this allows the meat to rest and keeps it safe. They then use charcoal to reheat it and give it some char. That's just one theory from previous food service experience.

Another thought is that they could be roasting the entire tri-tip primal cut as a whole piece overnight, then cutting steaks off of it and char-grilling them. Roasting the whole piece would take longer and it wouldn't dry out as quickly. You could do this at home too, provided you could get the whole piece of meat from a butcher.