How to Make an Encrypted File / Directory
Install p7zip-full from universe repository
Right click file or folder and select
Confirm archive name and select
Right click .7z archive select open with Archive Manager.
Select three line icon upper right, click Password to create AES-256 encrypted archive.
You will need to install 7Zip to open archive on a Windows machine.
If Windows is involved I would use NTFS or exFAT, exFAT needs to be enabled in Ubuntu.
When opening a file in the encrypted archive a temporary file is created. In Windows, if the computer crashes or the archive is closed before the file, the temp file may remain in /AppData/Local/Temp/, (%TEMP&). Everything is okay if the file is closed before the archive. In Ubuntu the temp folder is /home/USER/cache/. The temp file seems to be deleted with a crash or when the archive is closed. The User should confirm this regularly.
The encrypted archive will expand as required.
Selecting and remembering a strong password is an important part of encryption security. There are many good articles about this topic on the internet.
HDD's, SSD's and flash drives can all brick without notice. SLC, (Single Level Cell) SSD's and flash drives have the longest life spans. It is a good idea to keep a backup drive and copy of the password in your safety deposit box.
A full tutorial on P7Zip-Desktop can be found at: https://www.how2shout.com/linux/how-to-install-p7zip-gui-on-ubuntu-20-04-lts/