Advice about templates and using site_url in wordpress text mode

Hi,
I am fairly new to wordpress (Been developing in asp.net for 8 years) and I am just trying to understand the concept of templating.

I have a completed HTML website which I am trying to integrate into wordpress. I have setup wordpress locally using multisite so that I can work on multiple sites under the same wordpress working directory. So far I have been creating a template for every single HTML page so far and using the network_site_url to get the root of my current working website (which is “192.168.0.1/wordpress/” – as this is my primary website which is the same as 192.168.0.1/website1) via:

<a href="<?php echo network_site_url("Service/");?>" class="secondaryBtn">A link goes here</a>

However I am thinking to myself, this whole thing defies the point of “templates”. I should be able to just create a main template which all other pages use. If I create a default template and then paste my code for that page in the wordpress “Text” box next to visual then how do i add the network_site_url before the image or link? Would I have to find all the images tags for ex in the functions.php and then replace “/images/…”
with network_site_url(“Service/”) . “/images/…”

Thank you ??