Custom property editor; what does the property editor schema (alias) do?

I’m updating multiple Umbraco 13 packages to Umbraco 15. I struggle to understand how I can use property editor schema (aliases) in combination with Property Value Converters.

I have a property editor that allows for the selection of articles by id and when you do, it will allow you to select which content and video blocks you want to display on your site:

This property editor stores just the id of the article as well as the identifiers of the selected blocks. I have a property value converter that converts these id’s to a full fledged article with the requested content to be displayed on the frontend for both the models builder as well as the content delivery api.

Now I’ve built property editors before in Umbraco 15, so no issue there. But now we get to property editor schema aliases. According to this documentation:

It says:
You can perform custom conversion of the stored Property Editor data by implementing your own Property Value Converter. In this case, the chosen Property Editor Schema determines the input data for the custom conversion.

What does this even mean? I have a property value converter, so I guess it makes sense to not use Umbraco.Plain.Json but something custom? But how? There is no more documentation here that explains it?

And a secondary question: the Umbraco 13 sites will at some point be upgraded to Umbraco 15+. How will they know what property editor in 13 is the same property editor in 15? The data editors in Umbraco don’t seem to have the same alias anymore? Do I need to keep the aliases the same? Also, how will the property editor schema affect how existing content is parsed between Umbraco 13 and 15?