The security of a fully random password depends on the number of available symbols (alphabet) and the length.
The strength of the password is simply symbolcount^length.

For a conventional password the symbols/alphabet are characters, numbers and special characters.
For a mnemonic the symbols are simply full words and the “alphabet” is a list with a couple thousand words.

Mnemonic passwords are secure because of their large alphabet, and easy to remember because of the lower length (in symbols) and because human brains are good at coming up with associations (usually stories) for random words.
If you want to generate your own mnemonic password you can try diceware.
With diceware you roll a few dice to select random words from a list.