Can I still play online if I’m being game shared by someone who has game pass core

Game sharing outside your household is against the terms of use, and can get you suspended or banned for doing it.

Game Pass Core (at the least) is required to play online. A subscription has always been required. You may not have Game Pass, but you may have had Gold in the past (which was discontinued and replaced with Core) or someone else was sharing their subscription with you.

Game Pass Core or Game Pass Ultimate (which includes Core) is required to play online in most games. If you've never had a subscription yourself, it's likely someone else has used your console and set it as their Home console so you would be able to play online using their subscription.

If he has Game Pass Core, that would be the same thing (Gold was discontinued a few months ago and Core has replaced it).

If he has standard Game Pass for console, he can still use the card. It will give him the ability to play online.

If he has Game Pass Ultimate, he can redeem the code and it will be converted to 4 months of Ultimate.

Gold was discontinued and replaced with Game Pass Core. You still need Game Pass Core or Game Pass Ultimate (which includes Core) to be able to play online.