ok I have a paypal account and a joint credit card I share with my mother since I'm seventeen (worst age ever y/y.) We both have a card but only one bill which goes to her; there's a 500 dollar limit on it, but anyway
I've linked both my bank account and credit card onto the account so I no longer needed to pay by e-chequing. recently when purchasing some CSC I noticed that it had some gray text reading "if you have insufficient funds in your bank account we will take it from your credit card" or something to that effect; does that mean that when I buy online, regardless of whether or not I have a credit card linked, it takes from my bank account?
that's preferred, obviously, because I'm not really looking forward to my mom going 'WHO'S AGOGE INC AND WHY ARE YOU SPENDING SO MUCH MONEY ON THEM" but I'm just wondering. if so I wonder why I needed the credit card linked to begin with, haha

When you pay for something, you can choose the source of the payment. Usually it defaults to your bank account, but there will be a button that allows you to change to a card or another account etc.
I have both a card and an account linked because I have more than one account, so depending on what I'm buying, I pick either my card or my directly linked account. I guess they allow you more than one source option so that you have a backup if you make a mistake and don't have enough in the account you picked?
is right. The CC is a default for paypal to get their money in case you have NSF in the bank. You should be able to remove the CC from the account altogether or switch between which method you'd like to pay with.

It's a pretty good backup plan, that's all. So if you accidentally don't have enough they don't freeze you or give you a nasty unplanned overdraft. Ugh.