There's a lot here that feels like you're purposefully conflating to make it seem more confusing than it is and a lot of what you mention is also an issue in Ruby on Rails. It's also a false comparison to begin with. Ruby on Rails is more equivalent to Nextjs or Remix.
When I had to learn a little ruby on rails I found the convention over configuration far harder to wrap my head around. It's quite nice to be able to see for yourself exactly how pieces are wired up and configured instead of having it magically done for you and dictated to you that it has to be done this way.
There are standards in some of what you mentioned. Mainly hooks, fetch, npm, and css.
Ruby doesn't "solve" css either, you're just using regular css files which you can do trivially in React as well (<Foo style={myStyles}/> and define myStyles as an plain old javascript object wherever you want - same file or not) or just include the stylesheet on the server response.
The rest of it, state management, webpack vs grunt, etc is pretty simple to select from and even in all that there are obvious choices - redux, webpack, npm that are certainly considered standard.
With rails, there are really only a handful conventions, and while they take a little time to learn (table names plural, model names singular, controllers plural, name view files based on controller/action, foreign keys names in the obvious way) and that's about it. Once you have that you can look at a URL and know exactly where the code is. That is really valuable and worth a little effort.
There isn't much magic in rails, and just stepping through the methods in a debugger makes everything pretty clear if you do want to know exactly how everything is wired up.
When I had to learn a little ruby on rails I found the convention over configuration far harder to wrap my head around. It's quite nice to be able to see for yourself exactly how pieces are wired up and configured instead of having it magically done for you and dictated to you that it has to be done this way.
There are standards in some of what you mentioned. Mainly hooks, fetch, npm, and css.
Ruby doesn't "solve" css either, you're just using regular css files which you can do trivially in React as well (<Foo style={myStyles}/> and define myStyles as an plain old javascript object wherever you want - same file or not) or just include the stylesheet on the server response.
The rest of it, state management, webpack vs grunt, etc is pretty simple to select from and even in all that there are obvious choices - redux, webpack, npm that are certainly considered standard.