It doesn't matter whether it's a highly populated nation or not, an aged population or not. What the culture is, the religion, access to birth control, health care, income. For example, some places don't really use cars, think The Netherlands as a Western example, or poor agrarian nations.
The culture of Japan is very Western centric, yet also as far from any Western culture you can get. EG, high-tech, cities, cars, the trappings of "modern life", yet a culture entirely unique, wrapped in strong levels of duty, and with a different culture for woman.
Iran has a birth rate problem. A very rigidly controlled, religious nation, where women really have no say. Where people are paid to have kids, like many Western and other nations.
If you think "car seats" have anything to do with child rearing in much of the world, you're completely, US-culture out of touch. And further to that, my point is that every single culture, and whether rural farmers or people living it cities, it does not matter.
So whatever reason you want to give? Cost? Car seats? Contraceptive? Women's rights? Woke or not woke? Religion? Culture? Finances? None of it makes one iota of difference. None of it. Whether a poor or rich country? Nope, doesn't matter.
It's none of those things. Not a one.
If it was, then it wouldn't be all nations with a collapsing birth rate.
So to everyone reading this, know this truth. It's not economics, culture, the place you live, wealth, it's something else.
Something affecting us all.
It may not be microplastics. I could be something else.
But it's something which no amount of money will help.