I am trying to move from org development to source code development. 
i cannot find clear instructions on how to work with metadata. 

for example I declaratively created customapplication, objects, tabs, each object has page layout, custom field, etc. 
How to import it into my source code and update package.xml now? Import with all dependent fields, objects, etc?

if I import some custom object and try to push code, it fails as some relevant objects, fields are missing. 

I have no issue with Sfdx-project-cicd, but I don't understand how to import metadata with relevant objects, fields. 

Please some one show how to do with examples, not general phrases