DLC is typically installed along with various patches/updates that arrive alongside the DLC.
All that's left is a small bit of code that tells your system to "unlock" the DLC for you to access it.
ESO+ members get that last little bit of code downloaded automatically, with the caveat of an additional bit of code that allows the DLC to remain playable as long as the ESO+ membership is current.
Non-ESO+ members get the last bit of code once they go through the process of actually buying the DLC, and they don't have the additional bit of code that checks for an ESO+ membership.
So, the DLC is technically already on your system of choice, and all you're really technically buying is a couple lines of code that "unlock" access to the DLC.
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PAWS (Positively Against Wrip-off Stuff) - Say no to Crown Crates!
All that's left is a small bit of code that tells your system to "unlock" the DLC for you to access it.
Not even that. It's not a code base change (or if it is, I'd be seriously surprised,) It's simply account verification. Access is probably just gated server side. Otherwise you'd need to go through an update when you subscribed or when your subscription lapsed.