The stupid new forum software logged me out while I was editing. Ignore the above and go with this:
Welcome!
The SE had a SCSI (yes, we pronounced it "skuzzy" in the day) internal drive interface--they are getting harder albeit not impossible to find. And there are variants of SCSI with different connectors that can cause fit issues if used as an internal drive. Today even used working ones should be dirt-cheap because no one uses them any more.
One wise chap from Eastern Washington who used to be active here always suggested finding a original Apple 300e external CD drive like these:
:https://www.ebay.com/bhp/apple-cd-300
(I wouldn't pay more than US$10 for one.)
Its SCSI interface matches what's on the back of your SE. His recommendation was to remove the CD apparatus and install a SCSI-1 hard drive in its place--the connectors apparently are compatible without adapters. Then you install the system on the external drive and set it to be the boot volume with the Startup Disk control panel,
.
Before departing on this journey, PLEASE read up on SCSI, its evolution and how the connectors changed over time. It's more complicates that newer drive connectivity schemes and I'm pretty sure you will be dealing entirely with SCSI. Here's a good place to start:
https://en.wikipedia.org/wiki/SCSI
Getting all nostalgic here--my first Mac was an SE/30 bought in 1990 as I recall; we still have it although its HD is pushing up daisies. It's been 100 percent Macs here ever since.