If you're genuinely interested in this stuff, you kind of need to do some google research or an appropriate degree; it's complicated!!
A quick lay guide to how encryption works
here : the complexity and detail of the algorithms are probably something you'd want a mathematical doctorate before you get into.
As for why one password for your password manager is better than one password across all accounts. In brief...
Scenario one. Set up your manager with a strong password as per uk's national cyber security centre recommendations: three random words, like giraffetrampolinehedges. You'll be able to remember that: the cartoon pasted above is a good example. Plus, you'll almost certainly be required to implement two factor authentication, also previously mentioned. This means your master password is about as secure as it gets.
Scenario two. You use giraffetrampolinehedges as your password everywhere, from Amazon to Meta to the company you once bought a cycle top from. And one of them is either careless with your credential data (see the note about Meta, above) or (more often);you get sent a scam email which asks you to log onto a fake webpage, where you give away your password for free. Either way, your password is now compromised, and you probably don't even know. These passwords get sold for pennies en masse. I buy your details, and try the same credentials on your amazon account, to which you've added all your credit card details, and like most people you haven't turned on two factor authentication. Then I try a few other common accounts and get shopping in earnest. And if you've put the same password on your email too, you're stuffed.
It's a shame none of us gets taught this stuff at school but you can get good advice at
www.ncsc.gov.uk to protect yourself. In the interim, think about a password manager.
Declaration of interest: my last role was in cyber security. It wasn't, however, working for a password management company.