If you want a deeper dive, the chemistry here is actually really simple and (IMO) very cool.
I'm drawing on some really dusty knowledge here but TL;DR the cells can be modeled as two reactions, oxidation and reduction. One is gaining an ion and the other is shedding one. This is typically shown in the equation as some number of "e-".
Because there are two reactions, each one is described as a half cell and there are tables[1] of all the voltages that these half cells create. Therefore you can mix and match to create batteries of arbitrary voltages, though whether they're practical is another issue.
If you look at the table, you'll notice that some common battery chemistries stand out, like:
Li+ + e− ⇌ Li(s) −3.0401
for lithium Ion batteries (the other reaction adding the last ~.7v to reach the nominal 3.7 cell voltage you're used to). Similarly you can find the half cells that make up a 1.5v alkaline battery[2].
It can’t be quite that simple. I’m not at all an expert in electrochemistry, but you can put pure zinc and pure copper together in a sealed container with nothing else except ideal electrodes until the cows come home, and you will not end up with copper oxides, zinc oxides, or any useful energy output.