Concerning the public key cryptography for the exchange of private encrypted messages, I would have rather represented the shared public keys by padlocks, and the private key by the key allowing to open the padlocks. This way, it is easier to understand that there is a relationship between the two keys. In his drawing, 2 different keys can open the safe door, which is not possible in the physical world...

What about signed messages? Then the private key is the padlock and the public key is the... key.

Just to let you know, out of interest, you can have two different keys for a physical lock. It's how master-keying works.

I like this visual explanation of the Diffie-Hellman Key Exchange https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exc...

Yeah I've always liked this video explaining D-H using the same paint-mixing metaphor: https://www.youtube.com/watch?v=YEBfamv-_do