Cars get eligibility for updates depending on when they were purchased, what hardware they have (including incremental revs that happen with newer versions of a model), what features they have that relate to the update, whether they have purchased the (future) full self driving add on, whether they are in the Early Access Program, and whether the customer has complained to service about a related issue that the update covers... those are the ones I know about.
On top of this (or maybe under it) they have a layer with rollout tiers for Tesla owned cars, cars of employees who opt in for early updates, and customer cars. Probably more than just this. And then with all that they roll it out over time, so we're not all getting the update the same hour, but generally it starts with a trickle the first week and then becomes a flood of users getting a given update over the course of the following week or two.
If you're not on WiFi you might get the "Update Available" notice in the UI first, and then when you get to WiFi it downloads it. But it doesn't wait, if you don't see that notice and it has WiFi, it just downloads it.
Also Tesla says that if conditions warrant, it will download the update even when there is no WiFi. But I think it waits a while to try to opportunistically get on WiFi if it can, to reduce load on the LTE network it uses. If your car never connects to WiFi (which is up to you to do) then it will download, if it can, over LTE if the update is high enough priority.
There are probably silent updates as well as another commenter indicated. Don't know about those but it makes sense.
On top of this (or maybe under it) they have a layer with rollout tiers for Tesla owned cars, cars of employees who opt in for early updates, and customer cars. Probably more than just this. And then with all that they roll it out over time, so we're not all getting the update the same hour, but generally it starts with a trickle the first week and then becomes a flood of users getting a given update over the course of the following week or two.
If you're not on WiFi you might get the "Update Available" notice in the UI first, and then when you get to WiFi it downloads it. But it doesn't wait, if you don't see that notice and it has WiFi, it just downloads it.
Also Tesla says that if conditions warrant, it will download the update even when there is no WiFi. But I think it waits a while to try to opportunistically get on WiFi if it can, to reduce load on the LTE network it uses. If your car never connects to WiFi (which is up to you to do) then it will download, if it can, over LTE if the update is high enough priority.
There are probably silent updates as well as another commenter indicated. Don't know about those but it makes sense.