In the company I just left, I actually went through the process two or so months ago of migrating their Vercel deployment to AWS. I evaluated several options that are listed on the website and on GitHub, and we landed on using OpenNext via SST, it was a low-pain effort, especially given the CTO's desire to also migrate off of Next.js.
As other commenters have touched on - my understanding is the purpose of OpenNext is to package the output artifacts of a Next build in a way that can be deployed to a serverless environment, analogous to how Vercel does it. The supporting projects like SST and the other links in the repo are to take those OpenNext artifacts and deploy them to infrastructure generally in an opinionated way - additionally supporting some of the "extra" features described in the repository.
The last project I was working on was to then migrate from SST to Fargate, as a persistent process (serverful?) deployment was preferable for various reasons. In that scenario, we would just be running the built in server using the Next.js standalone deployment mode (effectively a `node index.js`). We didn't need the extra functionality covered by OpenNext.