This just inserts the code right?

I’m trying to follow some youtube tutorials on GTM. They say to install the code manually, however, I use a child theme and I don’t know how to insert the code via functions php file. There are other GTM plugins, but they have so many options and things I’m afraid they will interfere with the tutorials. From reading the description, it sounds like the only thing your plugin does is insert the GTM code in the correct places. It doesn’t do anything else, am I correct?