"How do you I ensure that everything is going to the external drive?"
Everything? Catalog and data sets?
I would recommend leaving the catalog on the internal drive, and just move the photos to the external drive, as it is the photos that take up most of the space. The instructions to move the photos are in "Part 2 — Updating Folder Location" of this document: http://www.computer-darkroom.com/lr2_find_folder/find-folder.htm
The reason you don’t see it in Preferences is that Lightroom Classic lets you store images on any local drive at any time, so a preference would be too constricting. There is no requirement to store all images within a single folder. They can be on any compatible local volume. I have Lightroom Classic track images stored on multiple internal and external volumes, because like you, I’m using a MacBook Pro with limited external storage. Most of my images are on an external volume, which Lightroom Classic has cataloged. But Lightroom Classic also catalogs a few images I wanted to keep on the laptop. Each volume is listed in the Folders panel, and in there, I can drag images among folders on the internal and external volumes.
If you want to control where imports go, you want to look in the Import dialog box:
In the Destination panel, in the expanded Import dialog box, you tell Lightroom Classic which folder on which volume (internala or external) should store the photos for the current import.
At the top of the Import dialog box, the Copy/Copy as DNG/Move/Add buttons also affect whether the current import is copied to the Destination and then cataloged, or cataloged in its current folder.
If you always want all imports to go to the same place in the same way, create and name an Import Preset, and make sure it’s selected every time you import. For example, if you always want imports to go to a specific external storage volume, create an Import Preset set to Copy files from the camera card to your Destination folder on that external volume. For future imports, all you have to do is look at the bottom of the Import dialog box to make sure your named Import Preset is selected. If it is, your images are going to the external storage volume.
As others recommend, my Lightroom Classic catalog is on my MacBook Pro, while the images themselves are mostly on the external volume. The catlaog only takes up a few GB so it will not use up all of the space on the laptop’s internal volume.
However, what you may want to manage is the Previews file for Lightroom Classic, which is in the same folder as the catalog. The more images you browse the larger it gets, and the Previews file (which caches previewed images to allow faster library browsing, a good thing) can become larger than the catalog file. But the Previews file is expendable, so you can trash it to free up space. Just be aware that the Previews file will grow again over time.