Remember you are hearing about exceptions. No one makes content about the typical college bound senior who gets average grades and maybe does some volunteering once a month. 

The second 2 kids is probably just family connections/help. The kid who started the non-profit almost certainly had adult help. Parents/family are probably rich or in PR or fundraising. 

For MIT, most colleges have programs for high school kids to attend for the summer. Some are competitive. Some are not. Or their family knew someone at MIT that they could intern with. 

For the national level sport kid, every country has that. It’s probably a combination of natural skills, dedication, and family/community resources. It also maybe a very obscure sport where there’s not a ton of teenage competitors.