To join more than 1 360 videos together and still export as a single but 360 video, Insta360 Studio is not going to be able to accomplish that. For that, and for free, you will need the DaVinci Resolve and a special plugin called KartaVR. I also learn that DaVinci Resolve 19.1.2+ will require you to purchase ($300!) it's full studio version just to use the Karta plugin. So I downgraded my to version 18. Good thing their official website support page still allow downloads of previous versions, but I'm backing up DaVinci Resolve 18 just in case, as well as the Reactor-Installer Reactor 3 for KartaVerse. If I have time, I'll try to figure out the highest version that supports the plugin for free.
Then, we just need to apply the kartaverse effects to the 360 video. The video can then be keyframed from the KartaVR controls in the Inspector section. These are the best tutorials for it I've found on Youtube:
#1. How to get KartaVR plugin and do keyframes in Davinci Resolve (note: resulting video is not 360, for that, check out video #2):
#2: Making a  equirectangular-panoramic video 360 by injector: Spatial Media Metadata Injector.
After getting the KartaVR plugin, the rest of #1 can be ignored, if the goal is to export 360 videos and not preframed video from a 360. The injector also works for images. Â equirectangular-panoramic media source comes from Insta360 Studio's export.
So to recap, to produce 360 videos, Insta360 Studio can only export single 360 videos/images in standard format (mp4/jpg), or  equirectangular-panoramic (equirect-pano) format. To merge multiple 360 media files into a single 360 video export, the Studio cannot do this (yet), so that's where Davinci Resolve (version 18, because the later version restricted this feature for paying customers) comes in. Using the KartaVR plugin, Davinci Resolve is able to read the source as 360 media properly and thus able to combine multiple media files into one single equirect-pano media (by switching off the KartaVR effect after applying the plugin to the files).
Now, this saves me tons of time exporting 360 to Youtube, because I can just be lazy and export a single raw file, instead of doing any special video editing, if all I care is more storage than presentation.