IIRC, once you've proven that you can reliably run a registry (which Google does anyway for their other TLDs), the application and fees for an additional TLD is much cheaper than you would expect (5-low 6 figures/year)
I think I remember hearing at DNW that the uncontested TLD application fee and annual fee are flat $185K and $25K. Donuts probably owns many more TLDs than Google, but I'm pretty sure they still paid the $185K fee.
In theory Google could serve their URL shortening service on the root of their .google TLD (links like https://google/xyz123).
Would probably break some apps which don’t correctly handle TLD-only domain names though, I suppose.
I would assume most fairly-tech-savvy people wouldn't trust such a link. It doesn't obey the rules for what we've been taught a real URL should look like.