This is a good question and, unfortunately, one that has a little extra complexity.
The simple part first: The Nik family of plugins, which Google acquired and then sold to the DxO company, were meant to be installed inside of your computer’s Applications folder. Since these plugins, like most programs, are stored inside of your computer you can use them no matter where you choose to store your Adobe Photoshop Lightroom Classic Catalog.
Fortunately, no additional steps are required to use the Nik Plugins just because you have chosen to store your Lightroom Catalog on an external disk.
What adds a little complexity to your question though is that the plugins that were included in the free installer, the one that you got from Google years ago, might not be compatible with the latest versions of the Mac Operating system.
Depending upon which version of the Mac OS you are using on that MacBookPro you might need to pay DxO for an upgrade so that these plugins actually work. See https://nikcollection.dxo.com/ if needed.