I know that these exist, and I know that they have filaments that are just shorted together at the base. I have the following questions:

1) Names:
Is it slimline? HO? IDK what they are called. Let me know…

2) Type:
Obviously they are instant start, but are they all HO/VHO or are there some normal ones?

3) T5:
I swear I have seen single-pin t5 tubes, but now that I am looking I can’t find them. Was this a thing?

Thanks so much!

They got the name slimline because early in their development they were going to use T8 tubes.  It was decided that 8-foot T8 tubes would be too fragile, so they went with T12 instead.  Slimline branding may have been a GE only thing.

Slimlines are not HO or VHO.  Although the tubes are the same length, slimlines use single pins and are instant start.  HO/VHOs use recessed double-contacts and can be preheat, rapid start, or instant start.  The 24-, 36- and 48-inch slimlines are essentially the same wattage as their bipin counterparts, although sometimes labeled 1 watt off.  For example, an F48T12 maybe labeled as 39 or 40 watts.  F24T12 maybe labeled as 20 or 21 watts.

I've seen T5 single pin tubes too.  I think they were commonly used in display case lighting.

I have a single pin T5 germicidal with plastic endcaps.
 https://www.lighting-gallery.net/gallery/displayimage.php?album=8026&pos=21&pid=245458
It is 3ft in length, labeled as TUV 36 and 40w.