Plugin insert code in theme question

I’m trying to make my first plugin which basically adds some defaults to a theme, but having trouble understanding one part.

I need the plugin to insert code into a couple of files in a theme, but I can’t find any info on how to do it.
Example: I need to add get_sidebar to single.php before the get_footer then I need to add a line to the loop before the excerpt call and change is_search to is_search || is_home