How do groups today sell without releasing the single first?

Stan culture

Competition....

kpop expansion combined with stan culture and competition

ready to buy whatever there is

the fans want to support the group...

Are sales really exploding though? People used to buy more albums back in the day, for the mere reason it was the only way to listen to your favorite songs whenever you want to. After that, piracy became widespread and the CD market collapsed.

Of course, k-pop is doing ok because the genre came with a whole culture around it ie. the stan culture. So fans' devotion translates into album sales. And the k-pop market became global so groups of today can reach an audience their predecessors couldn't.

Competition to be the first

Depends on what is your "back in the day". If it is 2nd gen, especially around 2010, this is kpop lowest point in sales, therefor any other era will look huge compared to it.

I didn't check but i don't think the behavior really changed, just overall sales being higher

The money is theirs? Let them do whatever they want.

Fandom, stan culture exploding

everybody wants to get ahead of the competition