The structure is an important point, for the life of the projects. Among my professional experiences, I was able to help on a project where the website was developed on NextJS, but the API was developed on C#. I won't say his name because I don't want to destroy his reputation.
This is where it gets really tricky.
NextJS is a framework used rather than React for its search engine optimization, its speed, and also for its included API. There are only a few reasons to use anything over a NextJS API.
Using anything else, make the connection between the website and the API really tricky as it has a different endpoint, and can be coded in a different language so you will stutter to find developers who know both languages perfectly.
Regarding the SEO of NextJS, the developers of the project used client-side rendering everywhere, which led to SEO problems with Google's bot, as it was not retrieving the page filled with its data, and on top of that, they used Redux...
Redux is, in my opinion, a pain. In some cases, you will need redux’s store, and in other cases, you will regret having it:
Imagine having a NavBar component, calling a redux action to retrieve the user's information and display it. And another component calls a redux action to get the target user's information. The data retrieved by the NavBar will be erased and replaced with the last data collected.
This also, as I said, led to SEO issues that had to be temporarily resolved by using a pre-rendering service that costs 💸 money. This is not good news for the site owner.
Please learn your fucking technologies before using them for real projects. Make sure you use the most optimized method to match any future functionality.