[Device Repair] R7027:
In other words "pin multiplexing" or "IOMUX"? This reminded me of "signal multiplexing". It's the same kind of idea, right? This is some old knowledge I recalled from my school days when I went to become an electrician. I don't know exactly what you call it in English, we call it "gymnasium". It's the later years of high school, it's what you might call "secondary education" I think, but instead of going for theoretical and prep school for university, I picked a practical path and started working right after that. Like I mentioned earlier in the linked forum post, we learned a lot of useful and practical things. One course we had was on how to set up everything, install cables, terminate cables, transmit and receive TV signals, do LAN installations, terminate fiber connections, etc. I was 17 at the time. Now it comes all back to me... I remember we blew up a lot of big capacitors for "fun"! Before the teacher comes, we would open the window to clear the air. But he always knew we've been up to no good. I'm in my 30s now and considering going for electronics engineering.

As I recall it, I2C is what allows us to flash or reprogram BIOS chips in-line on Gigabyte PC motherboards without having to de-solder them, right? This is like the replacement for UART? There is one more new standard I don't remember the name of now. Can you recommend some electronics kit or something where I can toy with this type of stuff?

This was very useful information! Thank you for sharing. I mean the whole segment on pin multiplexing. I didn't know this was possible. This is nothing short of amazing!

I don't know about These test points look like soldering pads, right? I have not examined the board in great detail, I will have to look at it again to be sure. But now that you mention this, you remind me that I was unable to find one specific test point on this board that it's supposed to be there. Could it be that test points are these flat contact points that have been painted over or covered by the blue stuff that the top cover of a PCB is made of, and that it needs to be scraped or scratched to make contact? Like a contact point that's protected and only made to be scratched to make contact when needed, like the emergency window breaker hammer on a train or tram? I can upload a picture so you can see what it looks like. I should probably make it a new post so we don't derail this post completely. Only signal name is given in SCREAMING_SNAKE_CASING_STYLE_LIKE_THIS (is this a thing for signal names?) but I think I have located the corresponding test point TP123456 (some number, I don't have the name right now). But I can't probe it with my multimeter test leads. I will show you.

Reading from my notes now. When I press the button, I see 3.55 V. If I let go of the button, it goes back to 0 V. This is on battery power, and battery measures 3.76 V. I'm not sure what side of the resistor I put the test lead on. I will do a new test and report back. This was done on board number 2. I have 4 of these boards, 1 of which is in a fully functional phone that will be disassembled soon (the others are all broken to some degree but they all died in the same way).

I found a discussion about the compatibility of Seek Thermal Compact camera on EEVblog forum. People were discussing different models, but the conversation started off with the Compact XR model and its lack of compatibility with Galaxy S9 Plus. From what I understood, the American version and Asian version of this device was somehow designed or wired differently, different from the global version and European version, which made it useless as a companion for various thermal cameras, including those from Seek Thermal.

Many found the solution in using some type of OTG dongle/adapter, and making sure to connect the adapter first, and then the thermal camera to that. The original poster reported also some funny finding, where he could sort of pre-heat or kick-start that thing in a different phone to get its juices flowing, and then moving it over to Galaxy S9 Plus and it would work just fine.

So I think you're right about current draw. Looks like they require more startup current than the phone can deliver. They used old model of camera with microUSB connector, mine is USB-C so it should work OOB but it doesn't. So I will try some kind of OTG solution before I return it.