Why was I so sure to express that if PINB was a variable, then PINB0 would also be a variable? (Probably, I created the definition from PINBn name without exploring/reading further!)
Now, I see (expetimentally) that PINB0, PINB2, ..., PINB5 are just constants and they return 0, 1, ..., 5 respectively.
Thank you for pointing my ignorance.