How to wire 3 core and earth 2 an led downlight

Hire an electrician.

What you assume is a live and neutral at the switch will more than likely be a live feed and the black wire will be the switched live, you need to find the downright which is wired 1st and put that into your pir supply and then wire to the downlights from the switched side

The question is not very clear, if the neutral in the switch coming in from somewhere but not going out anywhere. Then the neutral is useless unless you might had a smart switch which requires N. So now you will never have a permanent Live jn your ceiling until you connect the Live to the switch line together in the switch, use Wagos or a connectors block then a plate to cover the switch earth to be terminated with one connector, N, with another one and L plus SL both together jn one

The best advice most people will give is, hire a professional electrician who provides a minor works certificate. I am not sure from your description that you have a neutral at the switch, do not be confused by the colours, blue is not always neutral at a switch. There are mainly two methods that are in use for wiring switches 1. Ceiling rose method – only a live and switched live are at the switch. The blue coloured cable is being used as a live and should be flagged with a small piece of brown sleeve (this is sometimes missed off). Normally there is only one two core and earth cable in the switch. 2. Switch fed – this is where the permanent live and neutral pair appear at the switch. A switched live and neutral leave pair the switch. The neutrals are normally joined and not switched although some new switches do have a placeholder for this arrangement. There can be a variety of different cables coming to the switch. If you do decide to tackle this yourself, please make sure a cpc (or earth) goes to each light even if not used in the light. The circuit protective conductor (cpc) is there for a reason and it will fail an inspection if absent.