Plugin for replacing code in Admin files (help please)

Hello everyone,

I am trying to make a custom plugin that replaces some code of the admin files (index.php and/or index-extra.php from the admin folder) by a code specified by the user in an options page.

I know very little about plugins, and from what I understand, is not difficult to add a filter to replace, for example, values from the content of posts by any other words.

However, I don’t know how to create a plugin so it replaces some part of code in admin files like the ones I described earlier. Furthermore, I don’t know how to make a field in a plugin options page so the users can write their own code.

Someone knows how to do this or maybe a website or tutorial in the internet that can help me with this?

Thank you very much,

Antonio