I asked this some time ago, but did NOT got definitive answer. Can be those boatloader used in commercial Products like Arduino UNO "clones"?
Yes. The original author licensed Optiboot under GPL2, and it would certainly be painful to change the license since he disappeared. I'm not sure that it's clearly defined what GPL2 means WRT bootloaders, but I'd put them in the same class as Desktop Operating Systems - of course you can write commercial products that run on open source operating systems! If you make changes to optiboot itself as part of your commercial product, you should be prepared to release those changes as open source as well. Personally, I lean toward less viral licenses (MIT or BSD.)
Which kind of bootloader those boards from China are using as bootloadeR?
How could anyone possibly know that? Reports have ranged from Optiboot (but I don't know what version) (especially for Uno clones), Atmegaboot (especially on Nano clones), Adaboot (on Pro Mini clones, as per Sparkfun?), and "doesn't seem to have any bootloader at all".