Thanks for the explanation about "CBSubs Options". I checked the links you posted. Do I understand correctly that such an "integration" is both a "programmatic" extension (that adds these fields to the various data types) _and_ the accompanying UI part so you can edit those fields without using PHP?
Also, when taking a look at the "Plugin management" page of CB in my Joomla backend, I can't say I've become "less confused"

For sure there isn't a "CBSubs Options", but I notice there is both a "CB Paid Subscriptions" plugin (which is at version 4.8.0) and a "CBSubs Paid Subscriptions" which is at version 3.0.0 and which displays a warning saying it's not compatible with my current CB version (which I think is 2.11.0).
If I search for "CBSubs Options" in the "Install CB Plugin" page, then it finds it, shows me a nice description of what it is, along with a "subscribe" button which takes me to the Joomlapolis website about CB Paid Subscriptions - but I _have_ that. I'm afraid I don't "get" it. How does one install such "integrations"?
Sorry about the newbe questions. This is all far less intuitive than I hoped
EDIT: By the way: this "index.php?option=com_comprofiler&view=pluginclass&plugin=cbpaidsubscriptions&do=addplanstobasket&plans=1-2" was a very helpful tip. I can see that when I enter the ID of my new plan, it gives me a nice error page with "Amount not allowed"; if I fill in the ID of an older plan (which is still "published") then I get
exactly what I want to see: a summary of the order, with my billing address already filled in, and links to the payment processors at the bottom. This will help me debug much more efficiently!