Welcome! This is a community for all those who are interested in protecting their privacy.
PS: Don't be a smartass and try to game the system, we'll know if you're breaking the rules when we see it!
Cryptomater encrypts the file as soon as you add it to the vault, then the encrypted file is added to dropbox by Cryptomater.
Your Cryptomater vault is not the same as adding a file to dropbox directly. It creates a virtual mount where you put your files.
it's called Cryptomator.
when setting up a cryptomator vault, you need to tell it the vault's location inside of your dropbox folder, this will hold encrypted files, and then another directory in which you'll see your files normally.
after that's done, and you have also unlocked the vault, if you want to store files with cryptomator's encryption, you shouldn't use the dropbox folder for that, but the other one that I mentioned.
when you place a file in the second folder above, it'll be immediately encrypted and then placed in the dropbox folder. you can leave the vault as long as you need it, or even set it up for automatic unlocking. cryptomator does encryption for each file separately. its not like veracrypt that manages one large file.