I hope to shed some light on the zero が mystery.
I watched this specific video, and what Cure Doll means when she says that Japanese can have a zero が is that Japanese is a Null-subject language.
Null-subject is a linguistic concept that states that, for some languages, the subject doesn’t need to be uttered in order to exist. Japanese is not alone there. Many western languages are null-subject, such as Portuguese and Spanish, for exemple.
Quoting Wikipedia, “Of the thousands of languages in the world, a considerable number are null-subject languages, from a wide diversity of unrelated language families. They include Albanian, Arabic, Basque, Berber, Bengali, Catalan/Valencian, Chinese, Estonian, Finnish, Galician, Gujarati, Greek, Hebrew, Hindi, Hungarian, Italian, Romanian, Japanese, Korean, Maltese, Persian, Portuguese, Punjabi, Sindhi, Slavic languages, Spanish, Tamil and the Turkic languages, as well as most languages related to these, and many others still.”
If you are familiar with Spanish, you will know that “hablo”, means (I) speak. But contrary to English that require the utterance of the subject every single time, Spanish is a null-subject language, and you can drop the subject “yo”.
That’s the zero が in Spanish. It easy to notice the existence of this “subject が” in Spanish, because even when not said, it triggers conjugation.
(yo が) hablo
(tu が) hablas
(él が) habla
In Japanese, since there’s no verb inflection (conjugation) triggered by the subject, it is a bit hard to notice its disappearance.
By the way, the same idea can be applied to Null-object languages, which Japanese is also part.
That’s why 食べる is a fully formed sentence in the same way that “como” is a fully formed sentence in Spanish. Who eats (subject が) and what is eaten (object を) are invisible but grammatically present in both languages.
All I said is simply gibberish for native speakers of Japanese or any language, for that matter. Unless you have linguistics training, you won’t know if your language allows pronoun dropping or not. You will just naturally speak it. Ask yourself in your own first language if you knew its linguistic typology
And you don’t really have to know these things to speak languages… proof is that most people don’t know any of it, but everyone speaks at least one language.
I hope that it helps to clarify what Cure Dolls is talking about.