Answers in no particular order.

The names pre-assigned to lists are largely historic. The names pre-date the 10 custom lists. All lists can be renamed.

In Settings - Lists if you change the control at the top left to be List Mappings you'll see a table where fields are mapped to lists. For each item you also control how the field reacts to the list association. The out of the box settings for Composer do not auto-do anything. You can set Auto-Saves, Autocompletes enabled in the table. The checkbox items in the table can also be modified via a field's function button (if the field is associated with a list). For example click on the button to the left of the Composer field and have a look at the menu.

Album Artist does not have a dedicated list as most people (myself included) uses a single list for Album Artist and the Artist fields. You can use a custom list, (rename it to whatever), and map Album Artist to it.

There is an action statement named Fill Application Lists which is typically placed in a batch process and will accumulate your lists. You might want to empty the lists first in Settings (or in the Batch Start inline action via the Application List Functions statement). As an example, the Create Database Utility can optionally fill lists using the statement.

Also ... the Get Info statement can be used to extract an entire list. If you replace the default list delimiter with \n you can write the list to a text file. The Application List Functions statement can be used to add items. So you could always read a text file and batch add items. (Typically people only do this where the list content is not a simple tag extraction such as Artist)

This worked very well and I have populated all those lists.