Comic trailers serve the same purpose as any ad for story based content giving a general premise for the entire show/movie/comic unless it's long running like a tv series then it's reflective of that seasons hook or a particular episode. In the sense of comics tho it's usually overall premise or a specific arc if the story is running for very long
Like when Tapas and Webtoons did them more often for premium or originals it was a base premise of the overall comic up to whatever point it was at using relevant scenes from the comic itself paired with either voice acting or just music
The Tapas youtube channel has them here:
and between videos and shorts on Webtoon's channel
you can kind of get a general idea to I guess "mainstream" approaches
I've never seen people suggest youtube for posting them tho since gaining traction especially as a new channel (in any regard really) can be very difficult so it's usually suggested to try and format videos for tiktok and post them there or using Instagram reels or if you must youtube shorts especially since using certain hashtags and taking advantage of short format content means having an easier chance at quickly grabbing someone with interesting visuals, giving them what would essentially equate to an elevator pitch length promo to get them interested in the comic and then sending them off if they haven't scrolled by already
If anything the main obstacle outside of making the trailers themselves is just getting folks to click through to your actual comic if not just engaging with and watching the trailer
Approaches to formatting trailers varies from person to person so sometimes it's tweening existing comic pages to move around a little or going all out and creating new animations but of course everyone's different and like anything ymmv
Also like Katz mentioned if you post on social media like twitter, instagram or facebook you can pay to have your post promoted. Success rates vary from how much you pay to what target audience you select but it's an available option should you choose to use it