Is iOS update for iPhone 7 Plus free of charge?

Hello from Mesa, Arizona, USA:

I have an iPhone 7 Plus.  I didn’t buy it. I got it from my sister a few years ago.  The iOS that’s now on this phone is “14.6 (18F72).”  It appears that the very latest iOS update for my phone is 15.8.5 and that this latest update can be directly downloaded and installed on the phone.  

My understanding is that this iOS update as I’ve described my circumstances above  (including that I didn’t even buy the phone), is completely free, no fees whatsoever, no-strings-attached, and I can legally download and install it.  Is my understanding correct?