Hi
Typically they will drill a hole through the wall into the house to run the fibre inside, they will usually run it inside to where you want it, up to a certain maximum distance, and fibre can usually be run around the side of the house. I don't think they would take it under the door as it would be prone to damage there.
You will need a plug socket within a metre of the ONT, and you will also need to have close by the ISPs router, or you can use your own. The ONT is screwed to the wall and you will not be able to move it after it is installed, so if installing into a room that may have a change of use later it could become inconvenient, i.e getting in the way of placing furniture or prone to being tampered with by the rooms occupant (an inquisitive child perhaps). I suppose Openreach would always come and move it for you at a cost.
You can of course place your router anywhere but it would need running Ethernet cable from the ONT to where you want the router.
Regards
Phil