This is one of the most basic tenets of electrochemistry.
The theory is available in all basic chemistry texts. A bit too long to go into it in a single post.
Simply put, every chemical reaction at an electrode-electrolyte interface leads to a potential difference the value of which varies with each reaction. Also, the reaction may be an oxidation or reduction. Each has a different sign of the potential . That is, the electrode may be positive or negative. This sign has no absolute sense. By convention a Hydrogen electrode is given a value of zero. all other electrodes are given values with reference to this Hydrogen electrode. So we have what is called Electro Motive series of elements.
A single electrode is called a half cell as it can be one half of a battery. Single electrode potential is something analogous to clapping with one hand. It is only when there are two half cells one can get a voltage that is usable.
You can randomly choose any two electrode systems and find the algebraic sum of the half cell potential to get the voltage that can be developed by combining these two into a battery.
This article gives the basic electrochemistry of cells:
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This article gives info on how it all began:
<a href="https://en.wikipedia.org/wiki/Leclanch%C3%A9_cell" target="_blank" rel="nofollow noopener noreferrer">Leclanch%C3%A9 Cell</a>
