I made a copay wallet with a chrome extension.
Do not use Bitcoin wallets that are Browser extensions.
So , if that first computer dies? Then my bitcoins disappear?
If you have no backup from which you can recover from, yes.
But then it means the bitcoins weren't actually "on" or "in" the first computer after all, if they can be restored to another place.... So confused.
Bitcoins are never on "your computer". They are *on* a key, and wallets are just a collection of keys.
-Why is there no username and password for a bitcoin wallet (on copay anyway)
A Bitcoin wallet creator can choose to add this functionality or not. You can even code up your own wallet if you wanted to.
-Can I really only access my bitcoins from one computer?
You can access your Bitcoin on any number of devices at the same time.
-If that computer dies, can I recover lost bitcoins with that passphrase?
No. You need the wallet file, the private keys or the recovery phrase.
I suggest that you download the desktop version of Copay to both of your computers:
https://github.com/bitpay/copay/releases