fallen29 Hi
It depends on the T4 based template you're using, each template offers some ready-to-use blocks/element due to the concept of template.
With T4 BS5 Blank template, some custom blocks like header 2 can be found here: /templates/t4_bs5_blank/html/layouts/t4/block/
This template doesn't override any element, it will inherit elements from T4 framework plugin:
/plugins/system/t4/themes/base/html/layouts/t4/element/
You can find some elements used in header-2 blocks here such as: logo, masthead, off-canvas, ...
In case that you want to customize or add new element, you can create a new folder element in this template path: /templates/t4_bs5_blank/html/layouts/t4/
It will help to keep you custom element safe when you update new version of template or T4 plugin.
Regards