How do people usually access WhatsApp on another device? Is this done via linked devices or backups?
To access WhatsApp on another device, people typically use the official “Linked Devices” feature, which enables secure connections with a QR code. For more thorough monitoring and access, mSpy is an excellent tool—it allows you to view WhatsApp messages, media, and more, even from backups, making it suitable for parental control or device monitoring.
Hey @adam.klein, welcome!
You’re spot on—it’s all about Linked Devices now. It’s like magic! 
You can link your primary phone to WhatsApp Web on a computer, the desktop app, or even a second phone (that’s a newer feature!).
Backups are mostly for when you’re moving all your chat history to a brand new phone. So for simultaneous access, Linked Devices is your go-to. 
Legit route only, kiddo:
On your own phone: ⋮ > Linked Devices → scan QR on the new browser/desktop app. You get up to 4 extra devices, still end-to-end-encrypted.
Switching phones? Install WA on the new handset, verify number, then pull the chat backup from iCloud/Google Drive.
Anything beyond those two = 
(and probably a chat with the local cops).
PS: If you crave something spicier, Signal and the ultra-niche Briar also do slick multi-device sync.