If you put extensions in the /tce directory, not /tce/optional, it will be persistent.
Extensions in /tce/optional will still be there when you reboot, but need to be manually activated each time.
You can copy extensions from one computer to another. You can copy extensions from a previous installation to a new installation on the same computer. They do not need to be downloaded every time.
You can upgrade to a new version, leaving the extensions as they are.
There will be notes with each release about what files may have changed.
Another option when upgrading to a new version, is to leave extensions as they are, save a copy of personal files, and delete /home/tc and mydata.tgz, and it will be the same as a new installation.