Is there a way to have a video using embed code from youtube or other partners to be inserted into the post instead of the featured image? So if the video code is provided, it shows the video and if not it uses the featured image?
If there is no support for this, can you point me to the right direction as far as which theme file do I need to edit to make that happen?
Sure, if you are using the Block editor (Gutenberg editor), you should be able to just copy the YouTube link into the content, and it should embed automatically.