You need to prepare much of this in advance, so you are in control of the process.
We had FTTP installed recently. I had drilled a hole through the cavity wall just below the soffit, and into the bedroom above a fitted wardrobe. I left a trace wire for the Openreach man to pull the fibre through. The 'external' box is inside, next to the ONT. There is of course an external wall fixing to take the strain of the long external cable.
The Openreach guy turned up exactly on time and left in a couple of hours with the connection already working.