how to make multiple paypal account and keep them separated

No need of wasting your hard earned money. Multiple accounts multiple problems and you can never afford to miss it .

the problem is that I have seen people with access to huge amounts of paypal accounts but I don't know how they do it?

Creating 100 PayPal accounts is no big deal if you approach is correct. Get yourself a bunch of ISP Proxies, and create no more than 3-4 PayPal accounts on each proxy. When you're done creating 1 account, remember to always rotate your IP before proceeding to creating 2nd PayPal on the same ISP Proxy.

Post in the marketplace that you want to buy ISP Proxies, and someone will ping you up for sure.