C# for partial hard real-time application

Is a thread/process with highest priority a real time one?

I am new to programming.

It is believed C# and Windows are not suitable for hard real time application.

I am trying to find a method to use C# for partial hard real-time application.

Suppose there are 10 threads/processes in a machine control C# program.

All hard real-time demanded actions, controls and calculations etc are programmed within one thread/process with highest priority.

The machine operation is repeated every 5 seconds.

Demanded thread/process will run 0.5 to 2.5 seconds every cycle depends on product size.

Is this thread/process a hard real time one?

Are there any factors like garbage collection and operation system functions etc which will interrupt or slow down this thread/process?

Will all other threads/processes stopped during the period of time this thread/process is running?