It seems like Google have paid $25 million for the .app domain. So, when & where can I start buying .APP domain name?
You can't, not yet, and perhaps not ever.
The site for Google Registry Services makes no mention of the
.app domain that they won at auction on 25th February 2015. Although, it is included on their blog post saying which domains they have applied for which suggests the omission is deliberate (though it may be because the domain was contested).
Of the roughly 20 TLDs that Google has purchased and stated an intent to make publicly purchaseable, only
.how is currently generally available.
.みんな are early in the process where people can "apply" and reserve trade-marks etc.
It may be that Google reserves this domain entirely for their own, internal use like with the