Binary upgrade from package


I am wondering what steps the upgrade through official packages of an
existing nginx process follow.
I am ujsing official Debian packages.

More precisely, when the new binary is installed through the package
system, what signal(s) is(are) sent to migrate rfrom the old to the new

Is it a simple stop/start?
Or does the package upgrade follows steps from a simple on-the-fly
By it, I mean the following steps:
1°) Configuration check, if unsuccessful upgrade fails
2°) USR2 to the old master
3°) Start of the new binary, if failed HUP to the old master process and
upgrade fails
4°) QUIT to the old master

Of course, no edge problem such as the rtsig method would be supported,
any check for the new master process working properly (spawning childs,
answering requests) since that is maybe beyond the scope of a simple
automated package upgrade script, and some of the checks might require
human eyes and expertise.

That would ensure no downtime when official upgrades (including security
updates) are done.
Is all that already implemented?

B. R.