SST
SST is a provider that is backing OpenNext.
OpenNext is an open source project that enables you to "split the Next.js output, enabling selective deployment to different targets such as AWS Lambda, Cloudflare Workers, or Amazon ECS."1
So basically running Next.js as a distributed application in a serverless environment outside of Vercel.
This makes OpenNext unique since it enables you to run Next.js in a similar way as it does with Vercel, compared to all other providers and self-hosted options.
Playing catch-up
A small drawback with OpenNext is that the team behind OpenNext as to play catch-up with the Next.js core team. When a new infrastructure feature is being added to Next.js it can take some time until it is available with OpenNext. So do not expect to be up-to-date with the latest Next.js features.
At the moment all major features for Next.js v14 are supported with the SST AWS adapter.
How does it work?
As stated in the intro: OpenNext transforms the build output of Next.js into a format that can be deployed to AWS.
After running sst deploy in your project directory, SST is creating all the needed AWS resources for you.
This implies that you have an AWS account that is properly set up.
Scalability & Pricing
Since your Next.js app is deployed to AWS, you do not really have to worry about the scaling of your application.
Depending on the application load you are expecting you then only have to worry about your AWS bill.
But first you have to build an app that has enough active users. For reference have a look at the SST article about an app with 1000 daily active users deploy to AWS at 6$/month. Not a Next.js app but gives you a rough estimate. The advantage of using SST is that you have a usage based pricing.