I already have these files stored in OneDrive. Can I encrypt them within the program or must I extract them and encrypt before syncing to OneDrive?You should remove from OneDrive >>> Encrypt them locally >>>> Backup the encrypted files.
Any solution is unsafe against other admins on the same system.That is where cloud systems fail.
When I close down my Computer and restart it, and go to THIS PC I find that the Drive M in which the container was mounted, has disappearedThat's exactly as it should be. At this point in time your files are excrypted within the container.
When I double click on my container I do not get asked for password. Instead I am asked 'How do you want to open the file?That sometimes happens with windows10 the first time you open a certain file type. Just choose veracrypt as the program to open the file and tick the ALWAYS OPEN WITH option. Windows will not ask you again, once you have set the default program for Veracrypt Containers.
modern version should use aes to encrypt.
gnupg or pgp are also useful tools