The personal word list, which is where your “Learn Spelling” items are added, is separate from the Hunspell dictionary that’s installed in Scrivener.
And no, none of the dictionaries or word lists are project-specific. Both your dictionary and personal word list are global settings affecting all the projects.
The dictionary you install in File > Options > Corrections > Spelling panel is stored here:
If you want to replace that Hunspell file with one you prefer, we have these instructions on how to change the downloaded Hunspell dictionary for a different one.
In comparison, your personal word list is found here as that wordlists.txt file:
We have tips for copying personal word lists to a new computer. It’s handy to have around since it can help you find that list if you need to edit it.
And, for editing it, MimeticMouton had these suggestions in an older thread.