First fix usually refers to all the work that takes place before you get to plaster your internal walls and ceilings; the exact definition needs to be agreed to with all on site.
Once the structure is erected and you have walls, a roof and upper floors in place, your plumber, electrician, and gas installer if you have any gas appliances (e.g. hob, stove), will start running their services.
There will be a lot of coming and going from these trades; it is their busiest time on your site. They may rely on each other for different aspects and if going direct labour you will need to be kept in the loop.
The work includes running all cables, wires and ducting through walls, floors and ceilings. This includes water, wastewater, electricity, gas and other electrics such as CAT6 cables for broadband/ethernet points. It also includes any extractor fan provisions for the kitchen.
It’s best to choose all your electric fittings by this stage so your electrician knows what provisions need to be made for any supports or casing so the finish is eventually flush with the plaster.
Your ventilation supplier will be running their ducting at this point as well, and any home automation systems you require – any smart home features such as security cameras or home entertainment systems.
Consider if you want motorised blinds or curtain tracks, making provisions for electrics and a casing for them. You can hardwire (can lift heavier weight) or get them battery powered with a solar options. These elements need to be investigated at this stage. Electrical standards are constantly updated and you must use a registered electrician for all work.
The plumber will have all of the ducting in place for the wastes in the kitchen, bathrooms, utility and other rooms with plumbing. This is where any changes in plan will have to be dealt with, e.g. if the ducting was not positioned correctly at the floor laying stages then there may be a need to start chasing (cutting) floors or walls. If you are at fault, in that instructions were not clear, this is a cost you will have to bear, i.e. pay plumber to rectify, but if you have a builder and clear sets of drawings they should have followed these and it is their responsibility to put right mistakes as spelled out in the contract.