Curious about access to sacred places in Islam?

Assalamu alaikum! I'm not Muslim, just someone fascinated by your faith and trying to understand it better. I watch a bunch of Muslim vloggers sharing their daily lives and trips to sacred spots like Mecca. They mention certain areas are only for Muslims-so I'm wondering, how do they check if someone's Muslim before letting them in? From the videos, there are huge crowds; it seems tough to manage. JazakAllah khair for any insights!