Two camels are sitting in the basement and are splitting [like e.g. with an axe] petroleum. One says to the other “Hey, it’s Christmas tomorrow!”. The other one answers" I know, I came here on my bike."

I did my best with that translation, but there are subtle hints that these things relate to each other, that would take way more words in English.

E.g. the “eh” widens the meaning to something along the lines of “It doesn’t matter because I already made the decision to come on my bike anyway”.

So I guess the joke is a bit that it’s the framework of a cohesive conversation, but it goes by so fast (3 quick sentences) that you kinda only realize that it was nonsense when it’s already over. It has dad joke vibes, too, only German jokes are more about unexpected content than puns.

Compare also (again doing my best on translation)

A ball rolls around the corner and tips over

Or the “poem” (cause it rhymes in german):

It was dark, the moon shone bright,
When a car drove lightning fast
Slooowly around the corner.
Inside people sat while standing,
Silently engrossed in conversation.
While a rabbit who was shot dead
Was ice skating on the sandbank.