This post has a lot of answers to your questions especially the “WHERE SHOULD I PUT IT” section (covers some placements for the ultimate bundle).
Yep, I assume you use it alot so the usual spot (In the webbing between thumb and forefinger)
RFID stands for Radio Frequency Identification, both sides of the NExT are RFID sides, One is a low frequency chip that operates at 125Khz , one is a high frequency that operates at 13.56MHz (You may know this a NFC) and both can be used for locks if you get a compatible locks.
100% not an expert on this chip, but I think it is mainly for login to the VivoKey ecosystem, plenty of info on this forum if you search around. The NFC side of the NExT might be better for this.
All 3 of those use the 13.56MHz band so you might get collisions if they are next to each other, I would not want them too close.