Ask HN: How do you build a "trust layer" for your platform?

I'm trying to understand how different teams handle platform integrity and user trust as they scale.

For those running consumer apps, marketplaces, fintech products, or any platform with significant user activity: How do you currently build your “trust layer”?

Specifically: -> How do you detect fake users, bots, device farms, or automated signups? -> What early signals do you rely on to identify suspicious behavior? -> Do you collect any behavioral, device, or network-level data to make trust decisions? -> How much of your stack is home-grown vs third-party? -> What worked well and what didn’t as you scaled? -> If you rebuilt your trust/fraud pipeline today, what would you change?

I’m trying to learn from real experiences across different industries anything you can share (architecture, failures, lessons, tools you wish existed) would be super helpful. Thanks!