How do exchanges handle their holdings?

Like the title says. I would like to know how big crypto exchanges handle their holdings. Are there any specific protocols to make sure no single person can access the holdings or are they more decentralized so that no single breach can grant access to the whole holdings.

I am also curious if there are any bigger examples of such breaches and what the results where.

I would really appreciate if someone could give me an eli5 typ of explanation if possible as I don't have much knowledge in this matter yet.

Thanks in advance :)