StefBloke wrote:
• I scrimmaged around on the internet and found a third-party place that contains links to obtain the actual latest download directly.
The official Apple methods fail.
App Store won't let me download it (current Mac too old).
I don't believe this thread was about downloading macOS Sequoia to a Mac that does not support it. The OP's equipment line says "MacBook Pro 16″, macOS 15.0". All 16" MBPs that I can think of – whether Intel-based or Apple-Silicon-based – were released on 2019 or later and are therefore compatible with macOS Sequoia. The OP's desire to download an installer for a compatible version of macOS is entirely reasonable and the question would be simply if they are going about it in the right way.
You say that you are trying to download Sequoia to a Mac that is not compatible with it. That's not supported.
I have only one fully-working, ancient 2012-era MacBook Pro running Catalina, and have acquired a newer model (albeit one with the irritating T2 chip/TouchBar but beggars can't be choosers) that was decreed a write-off by the genius bar techs. I stripped it down to bare metal, cleaned it up, hacked off a shoddily-placed capacitor on the logic board that was destroyed by dust build up, put it all back together and the thing miraculously came to life.
But of course, the previous owner who took the Apple-recommended route of buying a new one, had everything locked down.
Macs with T2 security chips and Apple Silicon chips support the anti-theft feature Activation Lock. If the previous owner did not properly prepare the Mac for a new owner, and the Mac is still associated with them, you may never be able to set it up properly.
You should take it back to the shop, and ask them to ask the previous owner to follow the steps outlined here:
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
If the previous owner does this, then the following articles may be helpful:
Mac startup key combinations - Apple Support
How to reinstall macOS - Apple Support
If the previous owner cannot be found, or refuses to take these steps, you're out of luck. Of course, because this Mac was a "write-off", you didn't have to pay anything or much of anything for it, right? So you wouldn't really be out anything by having to recycle it.