The Auto-Play Video Previews setting is advisory, sites delivering ads or video previews (Netflix) will ignore it.
For mobile, there is Vinegar that blocks Youtube very well for me. IMDB is more difficult. There is a guide to blocking inside the IMDB app here. Blocking in Safari would require a monthly fee to an ad blocker like 1Blocker or someone to write a script for Userscripts. At present, there are no free methods.
Desktop is easier, just install Ublock Origin then install these filers:
||imdb.com/tr/*pageHit$xhr,redirect=noopjs,domain=imdb.com
||fls-na.amazon.com/$xhr,redirect=noopjs,domain=imdb.com
*$media,redirect=noopmp3-0.1s,domain=imdb.com,3p
- Youtube should be automatically handled by ublock origin.