How much does it make sense to use a third-party server in conjunction with Next.js?

Question

Node.js

Greetings!
Say express.js or NeSt.js ?

Looking at the NeXt.js documentation, is the default NeXt.js server even more low-level than express?

Or, if you don’t use the Next.js server, then don’t use Next.js at all and implement SSR, SSG yourself using the same NeSt.js?

Are there any critical benefits of NeXt.js that no developer would implement on their own?
I would be glad if you point out the gaps that prevent a full understanding of the essence of Next.js, thanks.