What is the difference between a protocol and a platform?

Using a platform requires using code written by someone else.

It might be a tool (open source, perhaps running on your computer)

It might be a service (requiring reaching out to the code running on someone else's turf on an ongoing basis and for example presenting an API key).

But the platform code was almost certainly written by someone else.

A protocol doesn't require using code written by someone else.

A protocol is a structured communication procedure that has the intention of having multiple entities that can speak it.

Sometimes if there's a very prominent first mover, everyone else just makes themselves conform to that API, it becomes a kind of protocol, even though the initial mover didn't intend for it to be,

In that the others interpret it as a protocol and intend to have multiple speakers of it.

For protocols, you can always write your own code to speak your end of the protocol if you want to.

In things like Gardening Platforms I use the word "platform" to describe what others might call "protocols".