Yes, if you use Synaptic you see all updates. You can easily enable level 4-5 updates in Update Manager from the Edit > Preferences menu. I usually have them set to visible, but not automatically installed.
The Update Manager has a set of rules, where each rule specifies the level of the update. The name of the package that has an update is matched to the rules, and the level of the update is taken from there. So even with PPAs, the names of the packages are matched against the rules. The list of rules is in /usr/lib/linuxmint/mintUpdate/rules. Or, the source is here: https://github.com/linuxmint/mintupdate ... date/rules
It might have some additional ways to decide the update level, on top of the rules. I'm not sure. The source code is of course available for those curious, here: https://github.com/linuxmint/mintupdate