Regarding Business Central Online (Cloud) Instance:

It is standard when launching the Business Central App on a Phone or Tablet with the universal app, that the default production environment is used.  If you wish to make changes and test in another environment such as "Sandbox", then you can create a url, paste or launch from a browser to access it.  for example,

ms-businesscentral://<Environment Name>



Opens Sandbox and you can access the related companies for testing...