Google tag manager allows you to add tags to links to help you track different information based on your needs. Such as adding a tag to newsletter emails to know which email a clicked link was from.

I have my subdomains set up so that each subdomain has a folder in the root directory of the website on the FTP server. Since you’ll likely have a separate WordPress instance in each subdirectory, you’ll need to add the tracking code to each of these instances. The principle of tracking is that the code needs to be loaded. When it loads, it sends information about the visitor to Google’s servers. So, if you have a subdirectory with additional content, but the tracking code doesn’t load because it’s only on the main site, the subdirectory won’t be tracked, and only the main site will be measured. Hopefully, this explanation makes it clearer.