There is an app on android called Video Downloader to pirate videos from websites. Now, it works for almost every website except youtube. The problem with this app is, it has no adblocker and when you visit sites you should not visit without an adblocker, it just rains ads on you. So, is there a website equivalent of this app which can download videos from any website?
Try changing your Private DNS on Android to dns.adguard.com to block in-app ads. For Firefox/Chrome apps, just find uBlock Origin in their add-on/extension stores (blocks ads on YouTube Mobile in browser). I’ve used file2hd.com on PC but it doesn’t really have a mobile UI.