If I check the processes that are running in the Task Manager and find that only the usual process run every time I restart my PC, can I assume that my device is not compromised?
In other words, does a compromised device always show an alien process running?
No.
Scenario 1: A rootkit is running, which could hide any malicious processes which are running.
Scenario 2: One of the 'usual' processes which you referred to could have been compromised, and you might not able to tell the difference.
There are plenty of techniques to migrate malicious code into "legitimate" processes. Furthermore, just because you cannot identify a process does not mean it's not legitimate.
As such, just by looking at the list of processes, you cannot tell whether a device is compromised or not.
First of all, I fail to see how you could check how all the processes running are legitimate, even if the malicious process did show up in task manager. The list of "usual" processes would change every time you install any new software and might even change after some software updates itself. Additionally, you might have a hard time telling which processes are "usual", since any malware that has established persistence on your device would show every time you reboot, and you might mistakenly start considering it "usual" too.
Even if you could do sort out the trusted processes, any malware that leverages something as simple as "living off the land" techniques (which is basically just using preinstalled binaries like powershell, cmd.exe, mshta.exe etc to carry out malicious actions) would completely bypass your test.
Of course, there are more sophisticated techniques like which MechMK1 and Dan Landberg refer to but its actually really simple to defeat your (quite impractical) check.
