Threaded, with teflon tape, on the exterior side. Hand tight, no wrenching. Moderately
firm - far from heroic - hand tightening.
Slip on the internal, strainer side. No glue.
I like my drains to be fatter than my returns. My standard is a 1/2" return from pump, but a 3/4" drain to sump. Cheap flood insurance. Real cheap, but quite good.
My fave glass holesaw vendor includes instructions. https://www.glass-holes.com/
They tell me not to approach an edge closer than 1 diameter of the hole I mean to put in. Thus a 2" hole runs from 2 to 4" distance from the nearest pane edge.
Just drill your drain as low as you can (determined by diameter of the hole-to-be, as noted above). Then, establish your dead pool elevation with a standpipe between the bulkhead and strainer. A little piece of schedule 40 PVC pipe serves well on a 3/4" bulkhead / strainer rig. This way, it's super easy to modify water level - just cut off a little of that standpipe, or slap in a longer piece. No screw, no glue, just push it in and pull it out.
And, you'll never kick yourself for drilling your hole too high - it's as low as you ever could have made it.
If you wanna get fancy you can put a few smaller holes or slits at the bottom of the standpipe, to not just drain from the surface but also draw from near the bottom. This is more important in deeper-water setups, or with messy eaters like turtles.
Hopefully this is all clear. Other have success doing other stuff, but all this is what works best for me. I have screwed up most everything at least once. What I've given here is the emergent wisdom of a whole lot of messing around.