I shared my computer and it would really help if there was an option to access Joplin by using the same password used for encryption. My understanding is that the files saved on the disc are already encrypted so nobody can see what's in there.
That's not the case, your data is only encrypted on the server you're syncing with (Joplin Cloud/Dropbox/One Drive/etc)
@sarah welcome to the forum.
As @roman_r_m says, your notes and the files attached to those notes are not stored encrypted by Joplin whilst on your device / computer.
However Joplin can implement "End-to-End Encryption" (E2EE). When E2EE is activated Joplin encrypts your notes and attachments when they leave your computer and whilst stored on whatever service you are using to sync your Joplin clients. It prevents whoever runs your cloud storage service from being able to see, search, index your notes. Also if that account or storage service is compromised your note data is still protected.
Think of each client you have as an "End". From the point your new or updated notes leave one client to the point they reach another of your clients, via the cloud service, they are encrypted by Joplin.
This subject comes up very often and has been discussed at length on this forum. Have a search for "PIN", "app password", "passcode" or "PIN lock" or something like that. There will probably be quite a bit to read
This is the Joplin FAQ entry regarding why Joplin does not have local encryption or a passcode / PIN lock.