You can add "cards", i.e. the popups in the top right that show a short text and an "i" button. Those can be links to videos, polls, or external links (if you'r

You can add "cards", i.e. the popups in the top right that show a short text and an "i" button. Those can be links to videos, polls, or external links (if you're a big youtuber). However the text is limited, you can't set how long it shows, and you can have a maximum of 5. You can maybe use subtitles too, but you cannot set a default subtitle for your video. So there are hacky half-working workarounds, but otherwise youtube just told everyone who cares about their videos to go and kindly get fucked.