I'm completely ignorant of Muslim culture but there was a time where Catholics recited prayers and liturgy in latin and plenty of practitioners recited the sounds without really understanding them.

Source: my grand aunts saying "arapreme" where "ora pro nobis" would go (the former kinda sounds like an Italian word). Also probably related, "hocus pocus" is the "magic" expression "hoc est corpus" ("this is the body [of Christ]").

Learning how to read and pronounce arabic properly is part of the religion. As all prayers etc are in Arabic and you are supposed to recite them in arabic not a translation and wrong pronunciation can change the meaning so they are strict about teaching proper pronunciation. Though I agree with you about the 2 billion number being incorrect but it was being used a turn of phrase not an accurate number.

Not everywhere. I live in an islamic region (around 40%). Religious quotes in every other home, but maybe 2-3% of people know at least some arabic letters, not to mention words. You learn it only if you learn at madrasah or are hardcore muslim (not as in extremism, but as in taking it seriously). After you learn it, you have to listen to popular interpreters, because Quran is still hard to get right on your own.

I grew up in a Muslim environment and I know al fatiha and more. But I don't understand a single word of it or can't tell if what I recited is a word or a whole sentece. I believe 99% of the population in my country are the same. Also we can't read Arabic script.

What country did you grow up in? We all don't speak arabic but can fluently read the script.

Turkey

In my surroundings I would say roughly 90% of people reciting al fatiha do so based on sounds learnt by heart, they would hardly be able to explain the meaning of these individual sounds.