Actually, this solution is not related to OJS, but is related to hosting company and CDN service.
Cloudflare is Cloud Network Delivery (CDN) provided by my hosting company. They provide cashing and delivery optimization, and fortunately, they also provide different apps within the account, which integrate into the output OJS code when accessing the site over the CDN. I installed “Add HTML” app, inserted YouTube embed code, and set on which pages and where on the page I want it to appear. It was straightforward and it worked nicely. Nevertheless, my YouTube video won’t appear if the website is accessed bypassing the CDN, i.e. directly on server. I also added social media icons and “to top” button in the same way.
Just of note, embedding videos in the custom blocks works directly within OJS (I have one such video in my sidebar).
See the sidebar video embedded in OJS custom block here: www.bjbms.org
See the video embedded with Cloudflare on article page here: Surgical anatomy of microsurgical 3-level anterior cervical discectomy and fusion C4–C7 | Bosnian Journal of Basic Medical Sciences