ElectronicE
People wanting to make gadgets using Bluetooth is a big one and I have been using Nordic Semi line of BLE SoCs very succefully.
Most of the business lately has been coming from a couple of companies that I made some work for and keep requesting changes or more features. I had to disable my Upwork profile as I didnt had the rrsources to keep up with the projects coming from there, even after upping my rates a few times.
It was very good pay at the beginning but it hasn't keep up and now it's not that good (to bad either).
A few years ago I started playing with electronics (Arduinos, Raspberry PIs and stuff like that). After a while I started looking and picked up a few easy jobs related to that in Upwork as a way to do something different as doing the same thing for 15years can take it's toll.
Shortly after I was picking more and more advanced jobs. I picked a few big clients and moved them off Upwork. I'm now doing advanced embedded system programming and electronic design as a side gig on weekends and afternoons and making more money than my main programming job (which I can't seem to be able to leave).
And that's the story of how I found an alternative to my comfortable job. If I ever leave it I will never go to an office again, I will just expand my embedded freelancing.