It's a whole lot of nuts (but not much fruit)
It turns out that 'industry standard' HDMI CEC is more of a set of guidelines than a hard set of specifications which is why different brands don't behave well together. F**ers.
How did I discover this? A half a decade old Samsung TV + a not fancy Sony soundbar.
Getting the Samsung universal remote to work (TV settings) was only half the struggle but still easier than Marx.
Once you use a DVD player, unplug and replug a RaspberryPi, use the TV as a second monitor (and general messing about), at some point the TV magically forgets the soundbar and goes skipping off through a field full of dasies.
This 'Standard says NO' has driven lots of others to go up the wall and give up. But I have a stupid stubborn streak and didn't, spending quite a long time going through all the combinations & reading internet posts to find a solution.
If you unplug the TV & soundbar power cables, wait ten minutes or so and plug it all back in and on it may work again; If not, unplug TV & soundbar power and the HDMI-ARC cables. Wait, plug in the TV power and turn it on. Connect the soundbar's HDMI-ARC cable to the TV's ARC socket. Only then plug in the soundbar power and turn it on. Go to soundbar settings on the Sony remote and make sure it is HDMI /CTRL is set to on and also /TVSTB. Exit.
Go to Samsung TV settings and find 'universal remote setup' and set up 'sound theatre' to go through the IR on/off tests until it works, et voila! Even then I still need the Sony remote to control sound functions.*
The first time I worked this out, I did not write it down. Cue a few months later and the same situation with even more swearing and learning the same thing again but writing it down. AAAARRRRRRGGGHHHH!
The kicker? The 75% won't touch the remotes. She tells me what she wants. I'm the remote control.
* There is one gremlin as every now and then the TV forgets which HDMI channel it's on and the 'source' button on the TVSTB remote doesn't work so you have to use the Samsung remote. Once that's done everything's find and dandy again.