Whee, I found a way to make an embedded YouTube video start and stop where I'd like it to.

(1) Embed as usual, (2) make sure it works, (3) then go into the HTML editor by clicking </> at screen bottom. (4) Add the following immediately after the question mark that ends the video identifier:

start=272&amp;end=525;

Use YOUR correct number of seconds, obviously.

The first paragraph below works to launch and cease playing the video where I want it, playing from 272 seconds to 525 seconds. The second paragraph below shows the original code generated by Canvas from when I first embedded the video on a Canvas page.

WORKS (plays just the section I want)
<p><iframe class="lti-embed" style="width: 640px; height: 480px;" title="2023's Biggest Breakthroughs in Biology and Neuroscience" src="https://www.youtube-nocookie.com/embed/qt6BO-DV0pY?start=272&amp;end=525;feature=oembed&amp;rel=0" width="640" height="480" allowfullscreen="allowfullscreen" webkitallowfullscreen="webkitallowfullscreen" mozallowfullscreen="mozallowfullscreen" allow="geolocation *; microphone *; camera *; midi *; encrypted-media *; autoplay *; clipboard-write *; display-capture *"></iframe></p>

ORIGINAL (plays whole video)
<p><iframe class="lti-embed" style="width: 640px; height: 480px;" title="2023's Biggest Breakthroughs in Biology and Neuroscience" src="https://www.youtube-nocookie.com/embed/qt6BO-DV0pY?;feature=oembed&amp;rel=0" width="640" height="480" allowfullscreen="allowfullscreen" webkitallowfullscreen="webkitallowfullscreen" mozallowfullscreen="mozallowfullscreen" allow="geolocation *; microphone *; camera *; midi *; encrypted-media *; autoplay *; clipboard-write *; display-capture *"></iframe></p>

See also this earlier posted topic with a couple of false starts:
https://community.canvaslms.com/t5/Canvas-Question-Forum/Setting-Stop-Time-Not-Working-for-Embedded-...

@SaraD20 had a good solution at the bottom, but it had garnered no likes. Thanks, Sara!