I would list the research fields in Super Mario Sunshine (SMS) as following:
- SMS Geography: hitbox of surfaces (walls, grounds, roofs) and objects. Mostly for clipping into surfaces
- SMS course by zelpikukirby (Japanese video): フルーツ壁抜け 水面抜け
- Tools: Surface Gap by sup39, supSMSTAS by sup39 (outdated), sms-livecol by QbeRoot
- SMS Physics (fundamental mechanics): QF (Quarterframe), RNG
- QF: Pausing is weird in Super Mario Sunshine
- Petey RNG: Petey RNG explanation by Noki Doki, ボスパックンデレ度チェッカー (Petey odds checker) by naosan
- SMS Physics (movement): movement in SMS
- e.g. The many kinds of Spinjumps by AverageTreyVG
- Many resources on X/Twitter by speedrunners and TASers
- SMS Biology: behaviors of enemies and objects
- e.g. The behavior of Eel Research by sup39
- Tools: sup-smsac by sup39, sms-tools by Goldfire711
- ACE (Arbitrary Code Execution)
- LOTAD SMS ACE Any% on PAL by Noki Doki
- RTA setup of TAS-only strats
- Many resources on X/Twitter by speedrunners and TASers
- Practice codes development for speedrunners
- Tools: GCT Generator, GCT Generator dev fork by sup39
- Challenges
- e.g. SMS Low-Nozzle A Button Challenge

Wow, I didn't notice how much research is done on SMS until actually writing them down. Note that there are still much work done by many people I couldn't list them all, especially those on X/Twitter since I have quit X/Twitter and couldn't access the resources anymore. In addition, I was mostly working on SMS Geography, SMS Biology, and Practice codes development, and I'm not familiar with other fields, so there might be some fields I didn't mention.

Anyways, thank you for showing interest on SMS. If you want to learn more, I recommend you to take a look at speedrun.com and join community's Discord server. Asking questions here is also welcomed!