for the pairing it sounds like you have the aqara sorted already - it recognised it as a “thing” you said , therefore just change the DH etc.
For the other one you have followed correctly how it ‘should’ work but I’ll explain how I do it, it’s crude but i’s relatively fast and takes just a few seconds.
Firstly to say this is my variant on ‘catchall’ method , which I won’t explain here as it is in these forums a 1000 times, but needless to say you need to be logged into IDE with the hub events showing on PC or tablet screen.
Press ‘+’ as you have, then using a pin press the button on the device as you have been, keep it pressed till you have 3 flashes then release, then press it on and off for about 20-30 times in a row, the light will flash several times during this. Then just go to the IDE screen and look for the ‘catchall’ entry which should have appeared (90% of the time it will have done), if not then repeat… ie press and hold for 3 flashes etc. NB after each ‘3 flash’ a new network ID is generated so make sure you have the latest ‘catchall’ event (refresh PC screen etc)
Then just change DH to appropriate one etc .
Sounds longwinded but really is less than a minute per device and works for me v well.
For the battery , I can’t find a square button switch but the round ones are easy - looking at the back there is a slight gap in the outside rim, and what looks like a little area you can press, (on left hand side as I look at it), just hook your thumb nail under this so that you can press towards the button and turn at the same time - pull to the right with your thumb - ie anti-clockwise motion - and the cover just pops off to expose the battery.
I have no direct experience of Samsung sensors, and I know this looks a bit longwinded for setup but reading lots of comments from others Samsung devices tend to lose their pairing far more often than Xiaomi ones, a and my personal experience once set mine are very reliable.