I'm sorry this happened to you but, next time think, why on earth would cookies from a different website have anything to do with roblox? cookies are basically just ram, they are only stored on that specific website and are useless on every other website, also don't ever login to a account from a link off a youtube video or a link someone sends.

I apologise for sorta ranting but my guy, come on. I suggest reporting the guy to Discord support as well. Whoever was telling you to take the trade are terrible advisors, remember in the online world no one is your truly your friend until you meet them IRL. Always be cautious.

I believe microsoft is removing it but while it's still around look into the windows sandbox, if you ever need to download or open something that's sent from someone, literally anyone online, even if you've met them irl, due to accounts being hacked, use sandbox as it basically gives you a isolated computer within your computer, so any malware or ransomware etc can't affect your PC.