Display of content-type I've got custom content-type with several groups and fields. 

I want to change default display of that content-type. In which file can I find appropriate code?
You need to create that file - copy the node.tpl.php file to 

node--your_content_type.tpl.php


and take it from there. Here's the relevant page from the handbook for Drupal 7

https://drupal.org/node/1089656

Alternatively, if you're just getting started with node templates, you may find it easier to install and enable the content templates module so you can add and modify node templates from the administrator backend:

https://drupal.org/project/contemplate


  The Content Template (a.k.a. ConTemplate) module allows modification of Drupal's teaser and body fields using administrator defined templates. These templates use PHP code and all of the node object variables are available for use in the template. An example node object is displayed and it is as simple as clicking on its properties to add them to the current template.