I've check over both the IllustrisTNG Public Data Release paper and the FAQs, but I just wanted to test my understanding of the distinction between subhalos (subfind group, subgroup) and halos (FoF groups, groups, etc.), can someone please correct me if I am wrong?
Subhalos are the subhalo for individual galaxies. Whether a galaxy is in a group halo or isolated, each galaxy should have a subhalo. On the other hand, halos refer solely to the halo of the group. In the case of an isolated subhalo, the subhalo should be the halo. Furthermore, the subfind merger tree algorithm identifies mergers by baryonic content subhalos (aka galaxy mergers) and NOT by a group (halos) metric.
There is a clarification here in the FAQ.
Otherwise, I just add that there is no such thing as "galaxies". The most common definition of a galaxy would be a "luminous subhalo", i.e. a subhalo with some stars.
I saw that FAQ but I think what I was missing was the blurb you just said. I ask this because I just got to a point where I am looking at the merger trees for post-merger dwarf galaxies (below stellar mass 10^10 <Msuns) in TNG100-1 and finding that there's essentially no SFR enhancement or starburst and I just wanted to see if there was a chance I was misinterpreting the merger trees or something like that.
I'm not aware of any studies about starbursts with TNG in particular, but for this topic you should definitely read Sparre+15 and Sparre+16 if you haven't already. And, you should definitely look at TNG50, given its higher resolution.