Here is my system as the macbook pro I got in 2016 has a relatively small SSD.
I use Lightroom as the database catalog for all my images.
I hook up two external hard drives to my MBP.
Hard drive A contains my working Lightroom catalog. Hard drive B is my backup to that.
When I go out shooting (RAW images) I come home and put the SD card into a USB card reader and use Lr (Lightroom) to import the image files from the card into hard drive A. I rename my images but won't go into that here.
Lr puts the images into the folders I've chosen. Real physical copies of those RAW files are now on hard drive A.
By keeping my images in hard drive A, I haven't filled my computer's internal drive.
The MBP and its USB 3 and Firewire connections are so fast that I don't notice any slow down working on images in a combination of Lr and Ps (Photoshop). When I save my finished works as Ps psd files and tif files they are in the same folders as their RAW files.
Lr keeps those psd and tif files viewable in its catalog, as I create those psd and tif files from Lightroom's export command.
By exporting this way to create the psd and tif files, those files have embedded the edits I had done in the Lr Develop module.
One needs to remember that when we edit in Lr Develop, those are all imaginary changes. The real image files haven't really been edited. Lr keeps track of those edits and we "see" them only in Lr.
Exporting embeds those changes into the exported image file.
Later I back up those folders from hard drive A into hard drive B.
My Lr catalog of course encompasses many more external hard drives than just hard drive A. I've filled a few external hard drives over time. I can hook those external drives up to the MBP any time I want if I want to edit/export from those other drives.
I can choose to put those exports into the same folder in which the RAW file lives, or I can choose to put those exports into any other folder on any connected drive I desire.
The party line on keeping Lr able to find all these photos is to ONLY use Lr to move files around.
There are ways around that but when new to Lr it pays to follow that advice.
I hope this helps some.