In some ways, it can be, but in other ways, it may not be. It’s not as black and white as one would think.
Normally in all circumstances, downloading content from a public or private platform can infringe upon copyright laws. For YouTube, this would go against their terms of service. But at the same time, there is no real law and enforcement of stopping users to download content for their personal use.
There is no clear, explicit law that would stop someone from doing so. Technically if one were to download content and then distribute it or share it online, then in all cases that would fall under direct infringement of copyright laws.
But downloading for personal amusement and use can be harmless as long as it’s intended specifically for that purpose and nothing else.
In any case, caution must be applied to avoid getting into any legal trouble. But with apps like Airy, your downloads are free of ads and copyrighted material, so there is no worry about copyright infringement in that case. It’s up to the user to be responsible and respectful about the content they consume and download.