Q - So the clone should be a "bootable version"-- does that mean there exist clones that are not "bootable"? if so, why would anyone ever want a non-bootable clone?
A - When a Clone is made with the third party software and made Bootable it means if one restarting the computer and use the OPTION key immediately at startup and with the Bootable Cone drive attached, the Startup Manager invoked by using the OPTION key - will see the external Drive as a Bootable Drive and can boot and run the External Drive as if it were a normal Internal Drive.
A - If that same Clone is not a Bootable Drive - the OPTION key will not allow the Startup Manager to see the external Drive as Bootable and thusly, can not boot from it.
Q - You said the clone "can under certain extra- ordinary circumstances can be used if he Internal Drive fails or be used to Reinstall the Clone Back to an Internal Drive" --so does upgrading to a different OS run an extra risk of having the internal drive fail?
A - Under normal operations - any upgrade to a newer version of macOS has risk factors. That is why one needs a Rescue Plan as outlined the following - Do I have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup is very useful and can be used to Revert to Previous Working macOS. For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site
Q - And if it failed, how would you know whether to start using the clone, or to instead reinstall it back to an internal drive? (i am guessing that to use it would mean to keep the external drive permanently plugged in to the laptop, whereas to reinstall it would insert its info and then you could detach the external drive?)
A - A Failed upgrade to a newer version of macOS would present various immediate indicator including the computer not rebooting after the upGrade process has attempt to finish or after the upGrade has completed.
Q - Is the internal drive the actual apple computer that runs my laptop? is it like the brain, and my physical laptop with keyboard and trackpad and screen etc like the body?
and would different OS versions like catalina and sierra be like grades in school, so if i install catalina it's like putting my mac into a higher grade?
A - The Internal Drive is where the Files including the Operating System is housed. The Brain as per say is the Central Processing Unit ) CPU where all the instruction are performed and passed of to the Operating System ( OS ) The trackpad one of the means to interact with the OS as would be a mouse and the keyboard. The Screen is only where all the Actions one has invoked are displayed.
A - Upgrading to a newer version of macOS like from Sierra to Catalina adds features added into Catalina that are not present in Sierra so in a way to a Higher Grade pus Higher Security
Q - Do you know of any good free third party software i can use to create the bootable clones? And is an ordinary flash drive okay as the external drive, or do i need a special kind?
A - Free is what I avoid at all costs. The old adage " You get what you pay for " is very true. Pay Zero get Zero and in some cases Free and get a lot of problems. There are Free Trails for some this party Cloning software. It can be used for a limited Time period and it works but one the Time Frame for Free Trial passes - it stops working or one has to Pay to keep using it. The two I know of , Superdupper and Carbon Copy Cloner. Personally I have and still use one of the two mentioned.
Q - And how do i keep my dozens of applications? (eg, things like firefox, spotify, adobe reader, etc...........)
A - Like any software what is installed Now on Sierra may not work on Catalina or may require upgrading. Sierra supports 32 bit applications, extension and drivers. Catalina ONLY Support everything 64 Bit and there is No Workaround period.
To check for 32 bit Applications - a good application to use is Go64 - safe to use.