Why are some SMS-recipients reachable through iOS but not macOS? I can launch the Messages app on Catalina and send an SMS to anyone on my iCloud or Gmail contact lists, which gives me the convenience of touch typing replies, as well as seeing attachments on a large screen—except for one contact.
This one contact, with whom I communicate often, uses an Android mobile phone. I had understood that so long as I am using a recent enough iOS version (it's 14.1) and a recent enough macOS (10.15.7), and am logged into both using the same Apple ID, I should be able to message anyone on my contacts list using the Mac's keyboard, regardless of the mobile OS of the person on the other side, indeed even if the other person uses only a very basic phone device.
When I enter this recipient's name in Messages, I only get a few older messages. When I receive messages from this one individual, I can only see them on iOS, not through macOS.
Why are some message-recipients reachable through iOS but not macOS?
In case it matters, this one contact used to use an iPhone, but moved to Android. Yet even when using iOS, I was receiving &quot;green&quot;, not &quot;blue&quot;, message bubbles, signaling that the messages are sent/received through the phone company's networks, not through iCloud.
On your computer, open the person’s contact card in Contacts/Address Book
Verify that the phone number you are attempting to use is classified as something other than “iPhone” (e.g. “Work” or “Mobile”)
Ensure you don’t have multiple contact cards for the same person, and create one for them if one doesn’t currently exist

If that doesn’t work, it might be worth trying to sign-out and back in to iCloud on both your computer and your iPhone, after re-booting both devices first.
(As for the messages when they were previously using an iPhone, the may have had “send all messages as SMS” enabled, or they didn’t a reliable mobile data connection while messaging you.)