Can you get a virus by downloading videos from an app?

You shouldn't, but there are some creative hackers out there who might find a way. Just make sure you have the latest iOS updates.

That's a little bit tricky because it's kind of old and Apple has discontinued major iOS updates. However, Apple is still providing incremental iOS 12 updates when there are security issues. The last one was last September.

About the security content of iOS 12.5.5 - Apple Support