Where you want it. How should we know.
My outdoor one (grey box) is next to the gas meter.
My ONT is next to my network cabinet under the stairs where I fitted it as I asked the engineer of I could do it, I just passed him the other end of the internal cable.
Exactly this. There were some permutations that weren’t possible when installing at my house, for example I wanted the fibre to go behind the garage wall (flat roof) and openreach weren’t willing to put it there. In the end we compromised and it runs across the front of the house slightly more conspicuously than I would have liked.