@golgaloth So that's what each step is doing. It's telling your computer "messages from the website 'icosahedron.website' have locks that THIS HERE key fits into", "messages from the Internet Infrastructure Provider 'LetsEncrypt' have locks that fit THAT THERE key", etc.
And each message, only that website, that Internet Infrastructure Provider, could have sent them,
because only that website knows what is the secret, private, number to put a lock a message matching THIS HERE key.
only that Internet Infrastructure Provider knows what is the secret, private, number to put a lock on a message matching THAT THERE key.
And it's not like someone could just, /guess/ the secret, private, number. right? Everyone knows that you have to be Very Careful to pick big, hard-to-guess, numbers.