I want to thank you for your detailed answer and, reading it, I acknowledge that probably I was drowning in an inch of water.
In fact, to not get lost in the procedure, I moved all the files I wanted to encrypt into a Gdrive folder, I downloaded it and then, once saved on my computer, I deleted it from my Gdrive Cloud.
Then I moved all these files to Boxcryptor Drive X so that, as far as I understood, they are now encrypted. And, in order to keep them so, I modify them from the Boxcryptor Apps on other devices.
If I may ask one last question, regarding this:
Are you telling me I could call Boxcryptor to make other applications (such as MSWord) to encrypt all the docs I created within these apps? I'm thinking about this sentence (found in the link you pointed out):
EncryptByDefault (default false): New files created by applications in unencrypted directories will be written encrypted by default.
Because that would be amazing!
Anyway, many thanks for the clarifications you gave me and for the link (it's quite technical for me but I will study it in depth)!