How do I search all contents including code from admin?
For a quick solution, you can edit the View that powers the Content page (or create a new View, edit a different View, etc), and add the body field as an exposed filter. Combined with a LIKE operator that should give you reasonably good results (though it will possibly depend on what input format you're using).

If you want something more like what you described, i.e. something that searches multiple fields at once, you could install the Search API module, create a results View, and add a combined filter that will search across all the desired fields. This approach takes a bit more effort, but might give more accurate results than the first one, especially if you use (for example) Solr as the backend.