From mediawiki manual on hooks :

Hooks allow custom code to be executed when some defined event (such as saving a page or a user logging in) occurs.
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MediaWiki provides many hooks like this to extend the functionality of the MediaWiki software. Assigning a function (known as an event handler) to a hook will cause that function to be called at the appropriate point in the main MediaWiki code, to perform whatever additional task(s) the developer thinks would be useful at that point. Each hook can have multiple handlers assigned to it, in which case it will call the functions in the order that they are assigned, with any modifications made by one function passed on to subsequent functions in the chain.

A list of hooks sorted by function can be found here: hooks grouped by function.

In my opinion we will need to use the Page Rendering group and the Skinning / Templates group.

I also think that further study of skins and templates is mandatory as they seem the right way to customize / extend the interface based on active extensions. Will probably open a new ticket to document progress on this (T167589).

A description of each hook with params can be found here hooks.txt