Thanks, Limnos, for taking the time to construct that detailed reply.
Incidentally, my 2018 Mini with an Intel chip apparently won't update to OS 26.x, but is as up to date as it can be at Sequoia 15.7.3. The 2022 MacBook Air with an M1 chip is running Tahoe 26.2.
And the change-password window-let is slightly different between them, but asks for exactly the same information.
Also, when I get to the actual change-password window, then on my Air it authenticates me by fingerprint, so I can only guess at what password it's using.
I tried to do it from iCloud, which says that because I have Stolen Device Protection on, I have to change it from a device. I'll have a look at my phone; things should be clearer there.
Having done that, it took me to another screen asking me a second time for a new password. (See?)
It seems to have worked the second time. I of course had to sign in again on the Mini and presume that I will have to on the Air.
Then it takes me to a window which says "Enter Mac Password - The password you use to unlock this Mac will also be used to access saved passwords and other sensitive data you store in iCloud."
And that is so obscure to me. Does it mean that I will have a different password for the Passwords app than for my computer or for my Apple account? Will the Passwords app password be the same as my Mini, on all my devices? That has not been my experience, and it seems that, for the Passwords app, I have to enter my device password individually on each device (which is good and fine), but it doesn't say that.
Then my phone says that I have a new device with access to iMessage (my Mini). Good! And then an identical message but about FaceTime.
I did it all over on the Air, and it needed sometimes my Apple account password and, also to resync iCloud, and for the Passwords app requires the device password, not my Apple account password.
I think I've got it all straight now, but I forgive myself for being confused.
Am i the only one on the planet like this, do you think?
Both computers also then asked for 2-factor authentication: good.