Face ID parts are complicated and difficult to repair. To fully understand Face ID, you can check below.
[guide|128388]
And for a detailed repair process, you need to pinpoint the faulty component first. Whether the flex cable has been ripped-off or relevant component has missing. Hope the following repair guide can help you somehow.
# '''Face ID not working caused by a broken infrared camera module flex cable:''' [guide|128681|How to Fix iPhone X Face ID Not Available]
# '''Face ID Not Available because of a missing component:''' [guide|136762|How To Fix iPhone 11 Pro Max Face ID Not Available - Motherboard Repair]
# '''Face ID Not Working After Screen Replacement - also flex cable broken:''' [guide|128932|How to Fix iPhone X Face ID Not Working After Screen Replacement]