How is someone gaining access to my devices and how do I detect if they are still doing so.

I have multiple data points that show I am definitely being monitored and have cast a wide net trying to figure out how they are getting in and exactly what they have access too. How do I figure this out? I know how to eliminate the issue by replacing devices/new iCloud accounts/router, etc. and reset administration of internet. I have been able to capture an IP address hanging off my network from another town nearby and couple other indicators but I really am at a loss how to KNOW when someone is accessing a camera or mic… because I live in a relatively close neighborhood I am also concerned that my cameras/mic or ones I am unaware of are on a separate Wi-Fi network or my network is going through a repeater nearby. Again, I KNOW I am being monitored and am very technical but must admit the days of C:\ are gone (simpler times reference). I am looking for a way to know for sure my house is ‘clean’ and make sure it doesn’t happen again once I replace all devices. I know this a tall order but if I receive enough input I can cobble together a plan that determines the ‘how’ and more data points for proof of the ‘who’ of which I already have some. I fairly sure I know the ‘why’.. the component I haven’t mentioned is they likely had physical access to my router, computer, iPad and iPhones for a short period of time. Concerned they have my IMEI, SEID, EID and certificate of my iPad. One event, I was told all my browser screens in safari are erased, checked and they were gone.. i have a lot more data points and proof but have already spewed enough. Thank you for any all responses.