While attempting to write my own script, I found that there are many websites which offer YouTube summaries, which are probably an easier solution. For example (not affiliated) https://www.easemate.ai/video-summary It even allows you to ask questions about the transcript.

I also found a Python library for fetching YouTube video transcripts, but some issue mentioned that they got banned, so out of caution, I implemented my summary script as a JavaScript bookmarklet instead. It will probably break on the next YouTube update, so I am not sure how useful it is. Also, you have to set your own API key (and maybe URL). I used Groq (not to be confused with Grok), because it is free and very fast.

    javascript:(function(){
    var GROQ_API_KEY = "YOUR_API_KEY_HERE";

    var btn = [...document.querySelectorAll('button')].find(b => b.textContent.trim() === 'Show transcript');
    btn.click();

    function checkTranscriptAvailable(){
        var transcript = document.querySelector('[target-id="engagement-panel-searchable-transcript"]').innerText;
        console.log("transcript:", transcript.slice(0, 50));
        var length = transcript.replace(/\s/g, '').length;
        if (length > 100){
            fetch("https://api.groq.com/openai/v1/chat/completions", {
              method: "POST",
              headers: {
                "Authorization": "Bearer " + GROQ_API_KEY,
                "Content-Type": "application/json"
              },
              body: JSON.stringify({
                "model": "openai/gpt-oss-120b",
                "messages": [
                  {
                    "role": "user",
                    "content": [
                      {
                        "type": "text",
                        "text": "Briefly summarize this transcript:\n\n" + transcript,
                      },
                    ]
                  }
                ]
              })
            })
                .then(res => res.json())
                .then(data => alert(data.choices[0].message.content))
                .catch(err => alert(err));
        }else{
            setTimeout(checkTranscriptAvailable, 1000);
        }
    };

    checkTranscriptAvailable();

    })();