Linux-next kernel version of the kernel tree
Is linux-next going to be the the mainline kernel later? If yes, is it the one first turns out to be RCs and eventually the stable kernel version?
Not quite; linux-next is described as
the holding area for patches aimed at the next kernel merge window.
It gives an indication of what’s likely to be in the next release, but a patch’s presence in linux-next doesn’t guarantee that Linus will merge it into the main tree, not does a patch’s absence mean that it won’t be included in the main tree. If you compare the linux-next and mainline trees, you’ll see that merges into the former are done by a number of developers; whereas merges into the latter are done by Linus Torvalds.
linux-next serves two major purposes: it allows subsystem maintainers to minimise the risk of merge conflicts during the merge window, and it allows developers to base their work on the very latest code queued for the forthcoming release.
Mainline isn’t built from linux-next; Linus merges pull requests from maintainers during the merge windows, from the maintainers’ trees. (This is finishing up right now for 5.19.) When the merge window closes, the tip of mainline becomes rc1.