fast startup is saved in the hiberfil.sys file on your ssd
If you unplug a PC or remove the ssd from the PC, it will not use the fast startup settings on the next startup. For fast startup to work, the drive needs to have constant power. Unplugging the PC will interrupt power and the PC will use the same power up method as previous windows versions, load everything from C drive.
Fast Startup combines elements of a cold shutdown and the hibernate feature. When you shut down your computer with Fast Startup enabled, Windows closes all applications and logs off all users, just as in a normal cold shutdown. At this point, Windows is in a state very similar to when it’s freshly booted up: No users have logged in and started programs, but the Windows kernel is loaded and the system session is running. Windows then alerts device drivers that support it to prepare for hibernation, saves the current system state to the hibernation file, and turns off the computer.
When you start the computer again, Windows does not have to reload the kernel, drivers, and system state individually. Instead, it just refreshes your RAM with the loaded image from the hibernation file and delivers you to the login screen.
Windows 10's Fast Startup (called Fast Boot in Windows 8) works similarly to the hybrid sleep mode of previous versions of Windows.
www.howtogeek.com
if you use an ssd you don't need it on.
If you move ssd between PC I wouldn't have it on, I have seen it corrupt installs from losing power.
it doesn't do any damage to PC to have it on. I have seen it catch some out when they left PC attached to power during storms and it hasn't booted next time, but that is more the storms doing than the modes.
When I will turn on the PC I will see the time on in task as PC just started? Then put back the old ssd with the OS with the startup enabled the system time on won’t be reserved and continue from where that was left.
if you have it off, time on is always since startup
If you have fast startup on, its time since the last restart operation
Restart in power menu actually turns PC off between the shutdown/restart action. It is only time a PC with fast startup is actually off.