Are lock screen pattern and system start pattern the same? I have recently bought a Nokia 5 and I am quite new to Android.

I have entered screen lock/and or start lock and entered a pattern. Acutally, I don't know if I did that one or two times but then I was very new to Android.

After some time, I have changed the lock screen pattern. Now I enter that pattern daily many times, but I didn't turn it off for, say, two weeks.

Now, after the installation of an update, the phone restarted and asked for a pattern to start Android. The lock screen pattern did not work. 

Since I entered the pattern too many times wrongly, the phone erased all data and I was forced to reinstall everything. Now I have entered a pattern (for lock screen) once again, and upon restarting the phone, the same pattern unlocked the phone.

So the question: Are lock screen pattern and system start pattern always the same or can the be different?
There is only lockscreen pattern that locks the phone for authentication. You don't have a system lockscreen separately. Same applies whether you are using numeric PIN as password based lockscreen.

If your device is encrypted, the pattern or PIN is the basis for encryption of key generated by system