How are subordinating conjunctions different from coordinating conjunctions?

Consider these sentences:

1. Tom reads novels, but Jack reads comics.

2. Although Tom reads novels, Jack reads comics.

I read somewhere that the first sentence is a compound sentence and the second, a complex sentence. But I didn't understand why that was true as they both are composed of the clauses: "Tom reads novels," & "Jack reads comics," and both these clauses can stand on their own as a sentence so they both must be Independent Clauses. Then I read that for the second sentence, one of the clauses we consider is taken to be including the conjunction too (which is "Although" in this example) and so apparently it becomes a subordinate clause (I am not clear on this too).

Now if this is true, then my main questions are: Why isn't the conjunction "but" included in either of the clauses in the first sentence? Why is it kept separate such that both of the clauses can be called Independent? Why is "Although" included in one of the clauses, deliberately making it a Subordinate Clause?

What is the reasoning behind this? To me it seems quite arbitrary and so I can't think of a difference between subordinating & coordinating conjunctions, apart from this artificially-created one.