Chemistry, please tell the master, I don't understand.

Assuming that the carbon-carbon bond in benzene molecule is alternating between single bond and double bond, there are two kinds of o-xylene:

In fact, there are only 1 species of o-xylene, so the assumption is wrong.

It means that all carbon-carbon bonds are exactly the same.

Five carbon atoms in cyclopentadiene anion are equivalent. Because cyclopentadiene anion has 6 π electrons, it conforms to Hooker's law and is aromatic.