zone system...

A really helpful book for me at the beginning was The Zone System for 35mm Photographers by Carson Graves. It got me to an "aha" moment, and after that the Ansel Adams book made more sense to me.

since we use roll film now instead of individually developing every frame, are some pictures going to come out okay and some come out completely over or under exposed?

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Another vote for Carson Graves book which actually tries to address the ZS for roll film photographers. What I do is load my own cassettes from bulk rolls with 8-10 frames max and use one cassette to cover a particular subject.
I will label the cassette with N-1/ N/ N+1 development based on the light measurements so I process each film/ session individually.
For 120 films you have 12 frames to cover a subject so again, no big problem in using a customized development based on your personal EI.
I wish you to have the patience to go though the entire process of testing and shooting ZS because this is where most people step back and look for shortcuts. It is tedious but rewarding.