I don’t know. My best friend has a Cannondale [I think that's what it is] and the front tire comes off easily for transport.

A foldable bicycle might be good for commuting to work, school, etc, but if you really want to use it for rough terrain, stay away from it!
Foldable bike will have incompetent parts compared to mountain bikes in the same price range due to the extra cost of the frame. Fold bikes also have smaller wheels, so it will be harder to go fast and will make the ride more bumpy. Fold bikes are also heavier. The advantage of folding bikes is that you will be able to fit it in tight spaces.
But first, it depends on what types of terrain you are going to ride on. I assume that you are a beginner, so a single-speed bike will be physically hard to climb with, but you won’t have to deal with shifting gears. It’s a trade-off. I would say that a foldable bicycle will be able to handle gravel paths, but not any serious mountain bike trails. If you are serious about mountain biking but don’t really know much about fixing bikes, you must buy from a bike shop. There they will have mechanics who can help you with problems with the bike. The guys at department stores don’t know much about bikes, so the bikes there may be assembled improperly. Riding on smooth gravel paths is perfectly fine with a bike without suspension, but again, suspension comes in handy with anything rough. Avoid full suspension bikes if you’re not willing to spend too much money.
So to sum it up, get it if you are riding mostly on roads and gravel paths. Get a real mountain bike if you want to get serious. You don’t want a cheap fold bike breaking on you while blasting down a bumpy hill.

Good luck on your quest for a new bike.

Second question: Some parts should be standard, but things like the fork might be harder to find.

folding is heavier