According to your first post you have a running Plone instance and you have Volto installed. Now you need to plug them together.
This is one way (replace Apache with NGINX if you want) : Cors Error even after changing buildout file : Volto - #10 by ksuess
For mimicing a deployment you build and then start in production mode:
Run a production build:
PORT=3000 RAZZLE_API_PATH=http://voltodeployment.example.com/api/ yarn build
Start Volto in production mode:
yarn start:prod
For development you do not build, you just start Volto:
PORT=3000 RAZZLE_API_PATH=http://voltodeployment.example.com/api/ yarn start
remark:
*http://voltodeployment.example.com/api/ * is your Plone backend with REST API and CORS configured.