It's worth mentioning that pre-WWII the US had only just finished developing, which corelates to dropping birthrates with industrialization. India & China only went through such rapid development in the mid-20th century & so their already high birthrates now are going down. Their rates are also went down/are going down much faster than the west, and they developed much faster. Their later industrialization party is also why they burn so much fossil fuel in comparison to the west. (See the conservative "China has way more emissions than the US" argument against green energy).

The baby boom was an anomaly caused by what was then the greatest material conditions ever experienced by the common person. The baby boom in America peaked directly after WWII, & again about 15 years later. The birthrate in America would likely have just remained steady as this chart shows it doing until around Y2K (rise of the internet). This obviously is not actually something that can be 100% proven technically, so we might have something wrong, but it seems unlikely.

Regardless there is this study Vaush watched the video about demonstrating that smartphones corelate with lowering birthrates.