I’m struggling to understand why some videos embed in our forum and some do not.
For example, if someone posts a YouTube URL it embeds as a playable video. Great.
URLs from other sites don’t render as an embedded playable video, even though I can see the same URLs as embedded playable videos on other (non-Discourse) forums.
So, are there restrictions somewhere? I don’t find anything in the settings.
Hello,
Not all video providers are supported by default. You can see a list here.
What are the sites you’re referring to? You can request it in feature if not available!
Thanks for the helpful response @Arkshine
So, supporting video embeds is literally done on a website-by-website basis? Discourse/Onebox has to choose to provide support for a specific website’s videos in order for this to work? Have I understood that correctly?
If you’re talking about generating a nice preview after pasting a link, yes, you’re right.
Every website has specificities, such as different formats, modes, URL structure, etc.
It’s not always as straightforward as wrapping the URL inside an <iframe> directly.
If Onebox doesn’t work with your site, this means no one requested it before, feel free to do so. 
Alternatively, you can still insert an <iframe> of any site directly into a post if you whitelist the source.
See allowed iframes setting:
I think you are confusing Create rich link previews with Onebox with embedding an <iframe> directly.
Creating a onebox requires the website to be supported internally by Discourse.
You still can post manually any <iframe> as long the source is whitelisted (see my answer above).
In both ways, you have full control over what to allow.
I hope that helps! 
Thanks for you help. I am a complete non-techy so I am finding this hard 
When I try to add something to that list I get this:
No worry! This setting is for onebox, to control what onebox is allowed to be rendered (among the supported provider I listed in my first message).
You want to use allowed iframe setting instead!
