As always with questions like this ... just try it! You don't have to save the results.
You will find that loading a PLA as a library will in fact load only the library parts... because no library can cause layers etc to be added. (Well, not 100% true... some attributes can be created by a master script, but highly unlikely a PLA from a client would do that.)
It doesn't make sense to load the entire PLA that contains the model as a library though... when you open the PLA you should have an option to save the library to a folder. Do that and then load that library folder (project library) as you move forward... cleaning up the objects in that folder as needed.
If working with a PLA, don't open, work with it, and then re-save as a PLA. The PLA is an archive format - not a intended as a working format.
This unpacking of a PLA is a really fundamental AC skill that everyone should know how to do... since creating a PLA of a project when it is finished is also a fundamental thing every office should do in order to archive everything required to restore the project.
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AC 27 USA and earlier • macOS Ventura 13.6.9, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB