You’re overthinking it. Every Apple device is signed into an Apple Account—that’s the name at the top of the settings. If the Apple Account paying for AppleCare One isn’t in that field, it doesn’t qualify. I have 5 members in my Family Sharing. My wife and kids have their own Apple Account so they would need their own AppleCare One coverage.
You need to read the print carefully—those prices are From a given amount. The $3.49 for a Mac is the price for a Mac Mini—not for ALL Macs. The ‘From’ price of $9.99 for iPhones is for the 16E— For Pro phones, it’s now $13.99 a month I believe, regular iPhones $11.99. Monthly AppleCare+ for a 13” MacBook Air is $6.99, for a 14” MBP it’s $9.99 and for a $16” MBP it’s $14.99. Same goes for iPads—that listed ‘From’ price is for the cheapest iPad (A16). For an iPad Pro I believe it’s $7.99 per month.