Your plumber will take the lead on installing your heating system in stages; whether or not there is underfloor heating will determine when they start the process. Your plumber will coordinate this with your electrician and do the commissioning.
For air source heat pumps, the installer will make sure there is nothing obstructing the free passage of air around the unit as per the manufacturer instructions. Short pipe runs, with good insulation, from the units to where the heat is needed will reduce the risk of heat loss.
Manifolds are usually associated with underfloor heating, i.e. pipes going to a room and back to the central panel, but can also be used with radiators. This is necessary to be able to control each room with an individual thermostat. Some systems rely on one thermostat per zone (7), covering multiple rooms, but the more thermostats the more control.
Stats need to be away from radiators or other heat sources, and shouldn’t be close to one another, etc. Your installer will be able to advise; on a new build you can expect a set of plans from them.