If you have installed the libraries through apt they will also be upgraded when all the packages in the system are upgraded.
PPA hosts the packages, including the library packages of KiCad, it doesn’t upgrade anything. apt update && apt upgrade does the upgrade for all available packages.
Apt, and other package managers, don’t know about which packages are programs, which are libraries, which are documentation, and so forth. All they know is when a more recent package, determined by version number rules is available. So yes, if there is an upgrade candidate, it should appear.
By simply typing “apt search kicad” you get an overview of all packages containing that string.
The lines starting with an ‘i’ are packages that are installed.
paul@medion:~$ apt search kicad
i kicad - Electronic schematic and PCB design software
p kicad-common - Old common files used by kicad - Transitional Package
p kicad-dbg - Debug symbols for kicad
i kicad-demos - Common files used by kicad
p kicad-doc-ca - Kicad help files (Catalan)
p kicad-doc-de - Kicad help files (German)
i A kicad-doc-en - Kicad help files (English)
p kicad-doc-es - Kicad help files (Spanish)
p kicad-doc-fr - Kicad help files (French)
p kicad-doc-id - Kicad help files (Indonesian)
p kicad-doc-it - Kicad help files (Italian)
p kicad-doc-ja - Kicad help files (Japanese)
p kicad-doc-pl - Kicad help files (Polish)
p kicad-doc-ru - Kicad help files (Russian)
p kicad-doc-zh - Kicad help files (Chinese)
i A kicad-footprints - Kicad footprints (modules)
i A kicad-libraries - meta-package for dep to all KiCad libraries (symbols,
p kicad-locale-bg - Bulgarian locale for KiCad
p kicad-locale-ca - Catalan locale for KiCad
p kicad-locale-cs - Czech locale for KiCad
p kicad-locale-de - German locale for KiCad
…
The above also implies, that if you installed some package not with apt but in some other way then they will not get updated ( Apt just a user friendly front-end for dpkg)
That’s not correct. It’s dpkg that keeps track of what packages are installed, usually in a small database. Apt and friends are a layer to add management of repositories, dependency resolution, searching and so forth. Sometimes there is yet another layer, a GUI interface.
A similar story applies to the rpm/{yum,dnf,zypper,whatever} layers.
With installing in “some other way”, I was thinking of manually downloading files and copying them to some directory. If you “install” files in this way, then dpkg has no knowledge of it and it will not be updated with normal updates.
(Doing things like this would be a horrible way to abuse your Linux OS, but that’s another story)
I though you were suggesting that dpkg would not keep track of packages. It does. Shhh! You shouldn’t even bring up unpacking packages manually or somebody might be tempted to try it. In any case using ar and tar to unpack packages is beyond newbies. If they do know how to unpack a .deb manually then they get everything they deserve.
Naturally copying from an Internet site or installing from a source build bypasses the package management system.
Edit: To understand how the layered package management came about you have to go back to the early days of Linux when installation was by means of a set of floppies passed around at user group meetings, and then later from CDs. The very first packages were just tar archives. Slackware Linux operated for the longest time like this. Internet access was by dialup, sporadic and expensive. So all the package manager had to do was keep track of packages and work from local package files.
When computers became more connected and all the time, it was such a joy to connect to repositories and manage packages by issuing an apt-* command which would get dpkg to do the lower level work. Server farms were huge beneficiaries. Today you can upgrade Linux hosts through releases with just a ssh connection and never having ever laid eyes on the servers they are running on.