I just tried it, it started installing without asking just like how its always been. So you should set which ever library you want as default.Can you pick which drive a game goes on? On my PC I will definitely want to pick what drive it goes on without having to move it myself (which kind of defeats the point as I will then need to navigate away from Playnite to the desktop Steam UI with kb/m to move it).
On my Deck it’s be handy to let me pick between SSD or SD too, but not a big deal as I can move it in the Deck UI.