When you install Logic, normally it just downloads and installs "Essential sounds" library on the first launch. If you're not OK with constantly downloading the sounds, most of the library can be downloaded and installed using the Sound Library Manager: Logic Pro X > Sound Library > Open Sound Library Manager.
Starting with Logic 10.4.2 there's also an option to relocate the entire library to an external HD. It's very useful if your system drive doesn't have a lot of space (especially in case your Mac came with a 128 GB storage). However, the library cannot be relocated to a drive which was designated as a Time Machine backup drive. After you relocate the library, you can still keep downloading and installing the missing sounds, and they will be installed to external drive.
If you're using an earlier version of Logic (10.4.1 and earlier), please, check out this article about library relocation: https://www.logicprohelp.com/move-logics-additional-content-secondary-drive/