what does it mean "sits top of NOLO"?
Is NOLO loaded first, or after bootmenu or bootmenu replaces NOLO?

what is then u-boot? what is a difference between two?

where in NAND these different boot loaders are located? /dev/mtd* ?

when selecting different OSes in either boot menu, does it just switch to right kernel and rootfs without doing any flashing?

(I know this info is scattered around t.m.o. but as the OP with search nothing thorough documentation could not be found.)