i dont think anyone actually cares unless you are in english class or someone wants to feel smug on the internet (you can ignore the 2nd one completely).
Informally, commas are "misused" regularly even by native writers, and almost no one notices or cares unless the misuse is frequent and egregious.
Formally, most comma use is an issue of style, not grammar, and style guides differ on when they should and shouldn't be used. The main rule that's actually a matter of grammar is this: commas cannot be used to join two "clauses" (phrases which could be independent sentences) without a conjunction.
English has no central authority, so the lines between grammar and style can get fuzzy. Most guides would say you need an "and" after the second comma, but some make an exception if the clauses are "very short and alike in form". See Wikipedia's article Comma splice if you are interested in the pedantics. https://en.wikipedia.org/wiki/Comma_splice
You still didn't answer: am I supposed to write "I'm eating a hot dog she's listening to radio" without a comma for that sentence to be grammatical? Or am I supposed to always insert "and" into it?
With or without a comma, without a conjunction like "and" the example is a run on sentence and is ungrammatical. You should join the clauses with a conjunction, and including a comma before the conjunction is a common style.
There's a caveat where some style guides permit using a comma without a conjunction in specific contexts, but most don't.
I don't know Japan but the author has a whole paragraph about how it's not the case, and how a big part of Japan success comes from the fact that this is not the case, so the author must be wrong one way or the other:
> Unlike other countries, Japan simply returned to the traditional private railway model of the nineteenth and early twentieth centuries: tracks, trains, stations, and yards were owned by vertically integrated regional conglomerates.
> There are substantial advantages to vertical integration. Railways are a closed system that has to be planned as a single unit. Changing the timetable at station A can affect the timetable at station Z; buying new trains that can travel faster might require changes to the infrastructure so they can reach their top speed, which in turn requires rewriting the timetables. This becomes especially complicated if different services share tracks. To prevent delays from propagating from one service to another, the timetable needs to be carefully designed to make best use of the available infrastructure.
> […]
> By maintaining and restoring the institutions that built the first railway systems in the nineteenth century, the Japanese have created the mightiest railway system of the twenty-first.
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