EDITED November 28, 2023 to add:

I resolved it, but only by purchasing a "known good" driver-in-kernel wifi adapter from the list at:
https://github.com/morrownr/USB-WiFi/blob/main/home/The_Short_List.md

The one I got was the "ALFA Network AWUS036ACM Long-Range Wide-Coverage Dual-Band AC1200 USB Wireless Wi-Fi Adapter w/High-Sensitivity External Antenna - Windows, MacOS & Kali Linux Supported" off Amazon (non-affiliate link) which was one of the few available as many of the chipsets included in the kernel are older and no longer for sale. But this one ticked all the boxes, came in at under $50, and when I plugged it into my Zorin box after booting it was recognized immediately and connected without a hitch.

So now it's in a box and on its way to BIL, who can now use it to test distros. Win/win. To all who responded, thanks for all your help!

First, my sincere apologies if this is a stupid noob question. I have a lot of tech experience but virtually none with Linux, so keep that in mind: I really have zero idea what to expect as I go along.

So I've been trying out multiple distros on my old mid-2010 MacBook, and have not had any problems at all: they have all seen my Broadcom wifi chip out of the box and just worked without a hitch.

On the other hand, my BIL (who heard about what I was up to and is now also trying out various distros via LiveUSB sticks I send him) has a MacBook Pro one year older, and NONE of the distros he's tried even see the onboard wifi. No wifi icon, no wifi in settings, it's like wifi doesn't exist. Ethernet shows up just fine, though.

When I looked into it further and had him do a specific lspci query to find out exactly what chipset he has, turns out he has a known problem: his particular MacBook Pro uses a Broadcom BMC4322 (432b) chip, which has only limited support under Linux via "wl" and maybe a "brcmsmac" driver written for legacy Broadcom wifi chips.

That's fine once he installs Linux, if he does, but right now he's just doing LiveUSB trials. We don't want to change anything on his existing hardware or HDD.

Okay, so maybe I can add some driver files to the LiveUSB or something? . . . nope. Not a good idea, because the other part of the whole fix is installing firmware, which has to be in place before the drivers will work -- but this chip is also still being used by the onboard Mac OS.

Needless to say, we can't do anything that might break his current Mac install. So anything involving firmware is not a good plan. Not only that, but I'd be doing separate drivers for every distro he wants to try.

Also, the house router is in a really inconvenient place, and without going into details, physically wiring him up via Ethernet isn't an option. If he wants networking, it has to be wifi.

So then I thought that since USB wifi dongles are cheap, we could just get him one, which would allow me to personally test it out and do whatever needs to be done on the driver side before he ever even sees it.

There's a little Netgear one that's under $40 that I have my eye on; it has to be physically tiny so he can still use the only other USB port tight up against it for the LiveUSB stick, and this fits the bill. They're handy to have, so even if he never goes full Linux we'd just keep it as a backup for ourselves. Win/win.

So here's my question for you good people. Keeping in mind he's still trying distros and has not even begun to settle on one, will a secondary USB wifi dongle allow him to test distros with wifi via LiveUSB sessions?

Are most standard USB wifi dongles supported out of the box by mainstream Linux distros?

Does anyone else have any suggestions on how to get wifi going via LiveUSB just long enough for him to try individual distros?

Many thanks for any help you can give.