Let’s go for a short history lesson to bring everyone up to speed - Hundreds of new TLDs, or domain extensions, became accessible to register beginning in 2014. These domain extensions include everything from business domains (like .coffee) to geography domains (like .london) to generic words (such as .world).
And now there's .app, the newest top-level domain (TLD) to enter the market, and it's a doozy. One reason for the popularity of the .app domain is very obvious, and it is because of the widespread use of apps. Another reason is that Google is backing the .app TLD.
Image source: Google
Google bought the exclusive rights to the .app TLD from ICANN at an auction in February of 2015. Three years later, they launched the .app gTLD (generic top-level domain) in May of 2018, rendering this answer and question kind of obsolete. First they had a little pre-sale/Early Access event where people could only buy the .app domains from Google themselves, but a day after that they allowed everyone to purchase such domains, even from other registrars.
Lastly, if you’re looking for a decent registrar to purchase .app domains from, you can check Nominus and their .app domains here - .APP Domain Registration