Answered a bunch of times... but:
1. 6" inch RC is a drop bracket kit, here's the link:
6 Inch Lift Kit | Nissan Frontier 2WD/4WD (2005-2021) | Rough Country
You need larger wheels because the OEM wheels aren't large enough-offset enough to clear the drop bracket parts.
2. Titan swap means you swap Nissan Titan front end parts onto a Frontier to get more width and more travel.
Please, use the search feature of the website for both of these, there's tons of information already explaining all the basics and more.