You may have noticed that Google sent out an email last week which caused a lot of confusion, because it said to upgrade to the latest tag within Google Analytics. This is going from analytics.js to gtag.js. Part of the reason it caused so much confusion is because obviously you want to make sure that you’re running the latest version of Google Analytics and people were concerned.
GTM has many pre-built variables off the shelf. You even have the option to create/ customize your own variables known as a User-Defined Variable in GTM. One of the User-Defined Variables I find very helpful is the Lookup Table variable under the Utilities section of the User-Defined Variables.