Comment by kevingadd

Typically deals like this are about a mixture of cash and access. The platform holder will often partially or fully fund development, sometimes they pay an advance on sales up-front. For one project I worked on Sony provided an up-front advance that fully covered the port in addition to some sales (and it took a while to recoup for reasons mostly under their control.)

My guess for this is that given Terraria's large fanbase and high profile, Google probably handed them an advance for this and promised some promotion once the title launched on Stadia. Stadia also potentially provides access to users who can't play it on PC (it has a client for phones and some TVs, etc)