Hi Andres,
I'm not a developer, but I can point you to some initial resources that should help you get started.
First, AtoM is a PHP project that was built using the Symfony 1.x framework. To understand how AtoM's code is organized, it is useful to understand Symfony's model and general architecture. We have a number of good resources for new developers linked from this wiki page:
That includes a link to the Symfony 1.x documentation. Since much of the files you will need are contained in plugins, it might be helpful to review the Symfony 1.x documentation on plugins, such as:
There are also two slide decks linked from that wiki page that should help give you an overview - see:
From the sounds of it, most of what you will want to modify will be templates. AtoM has some base templates here:
More specific templates are often a part of the plugin to which they belong. For example, the ISADg) archival description view page template - indexSuccess - is found in sfIsadPlugin, here:
As for making local customizations:
Much as in your previous thread, I recommend making these changes via a custom plugin if possible, because that way it will be easier to maintain your changes through future upgrades.
I am attaching a PDF we give to clients who want to install their own custom theme plugin - the important part for you will be the second part of the PDF, which explains how custom theme plugins are generally organized. You will see that you are able to include copies of templates that will override the default ones in AtoM if present. So, if you wanted to a a custom header to your AtoM site via a custom theme plugin, then you would make a copy of
this _header.php file in your plugin's template directory, customize the HTML / PHP as needed, and then also update the CSS classes affecting it in your plugin's style sheets.
I don't actually know if you can override another plugin's templates in a custom theme template (such as the ISAD indexSuccess template) - you will need to read a bit and experiment a bit I guess.
If you haven't found it yet, there is a sort of skeleton for creating Bootstrap 5 theme plugins in this public repository:
See the README for details.
In general, I recommend that you review:
I hope this helps a bit!
Cheers,