Hmm I am not sure what exactly is Amazon trying to solve for? I reckon only the managers know..
Moving to office-first does not solve the root cause - because it might be some other deep seated issues.
If it's really about boosting collaboration and communications in a remote working culture, that requires fostering by the Management especially the company does not start with remote first culture from the get-go.
These companies do the regular stuff such as advertising via the usual channels- job boards and LinkedIn. And sometimes with recruiters - not those fancy ones who recruit for start-ups.
Just sharing my own experience..I used to work for start-ups and now I am working for a seemingly boring, profitable company that is not venture funded. By boring I meant there is not alot of media news about them. They just bunker down and do the work. There's way less craziness - more focused, mature approach towards problems.