The logic behind rails' form_with

Any form, no object, just data to send, all you need is the target action route
       form_with url: posts_path do |form| 

Form for object, you add it **generically** to the tag.  But `@post` still needs to be defined in the controller action
       form_with scope: :post, url: posts_path do |form|

Form for object, safe for both post and patch paths.  define `@post` in the controller action
       form_with(model: @post) do |form|

Form for object, but you want to be lazy and not define `@post` in the controller action 
       form_with model: Post.new do |form|
   
Since you're using a form partial, you're possibly invoking a specific object on update:   
       form_with model: Post.first do |form|
or - in a more practical manner - and more to the point where `@post = Post.find(x)`
       form_with model: Post.find(x) do |form|

By definition, the form is HTML over the wire.
So if you want to redirect to a view...
       form_with (scope: :post, url: posts_path, local: true) do |form|

The rest is as in `form_for`

Rails shifted its stance between 5 and 6
As of Rails 6, to do UJS, you need to specify  
    local: false
true is on by default

Rails 7 default  is  turbo_stream, opt out via `, data: { turbo: false } `