Ask HN: How do you build referrals and incentives into your app in 2021?

Noticing that a lot of B2B apps (Superhuman) do referrals today, in addition to B2C (Airbnb) -- wondering if all companies building referrals into their app spend engineering resources on it? Or are there any common wisdom SDKs which make it as easy as integrating GA/Segment on your website.

If you've built this before, would love to know the challenges and iterations of doing this.