Slightly late response but here's my philosophy on it: if you're solely using Youtube and not clicking on links (and I mean absolutely NO links), you should be fine. However, you'd probably be wise to use a front-end like Freetube. You can disable comments and video descriptions, effectively turning it into an online video player only.
As for Discord, under no circumstances should you use it on Windows 7. I love that OS, I hate that I had to switch, but with all the threats and social engineering that occurs on Discord, it's just a bad idea, full stop.
One way you can help at least partially protect Windows 7 without software is to use a DNS resolver like Quad9 and enable DNS over HTTPS in your browsers. Quad9 is a private DNS option but they also block malicious sites by default, which can help work as an additional safety measure. Ideally, you shouldn't be browsing at all on Windows 7; just use it for older games, maybe for Youtube through Freetube as I recommended, but don't be using it for browsing. If you want to take your life into your own hands and do so, at least set your browser or ad blocker to disable JavaScript by default.
In general, as much as it sucks that we have to move on, Windows 7 is fundamentally insecure by virtue of the fact that it's completely unsupported now. Avoiding threats isn't always just a measure of having antivirus and avoiding sketchy links. Today, all sorts of malware gets through due to exploits and social engineering.