This kind of arrangement was relatively common "back in the day" and as such has a long history. In the 1970s for example Zeiss and Konica formed a relationship and some Zeiss designed lenses were made by Konica and branded as such. Konica also manufactured some Zeiss designs for other Japanese companies - the 50mm f1.7 was one such very successful design which was sold under many other high end Japanese camera labels, apparently made by Konica.
Leica did much the same kind of thing with Minolta. And sometimes the arrangement went both ways - when Leica wanted to step up its marketing of its SLR cameras they needed zooms an area in which they lacked design skills. So they did a deal with Minolta and initially used Minolta zooms (which for Leica's purpose were made by Leica using Minolta designs and glass) because Minolta had that particular expertise and Leica wanted to develop theirs . (A Minolta 35-70mm mm is a case in point which was sold by Leica in Leica livery as a Vario-Elmar).
Later when digital photography went beyond being just toys, Leica which was and is still relatively small when compared with Nikon, Sony Panasonic, et al and lacked the big bucks for R and D in the field, entered into an arrangement with Panasonic - it provided the glass (which was its area of expertise) and Panasonic provided the electronics expertise and technology. A number of Panasonic pocket cameras with Leica glass were rebadged as Leica cameras while much the same cameras were being sold as Panasonic with Leica badged glass.
I presume that Panasonic lenses branded Leica for 4/3 are still designed by Leica. Are they better than Panasonic lenses? I am not sure that I would put it that way because these days lens design skills are widely dispersed and almost every 4/3 lens I have tried is excellent or close to it (helped I guess by the small sensor). But the Leica badged ones are certainly excellent too.
For example one of my older "Leica" 4/3 lenses - the Panasonic / Leica 25mm f1.4 D Summilux lens made for the original 4/3 DSLR system (i.e. before mirrorless) which I still use with an adapter on 4/3 mirrorless cameras is about my favorite 4/3 system lens, being very sharp in the centre at f1.4, exceedingly sharp across the field at f2 and full of character. Is it better than other m4/3 lenses? Technically probably not, but it is certainly as good as I could ever want, Leica Summilux badge on the front or not. I am seldom swayed too much by considerations of whether a lens for this type of camera is badged Leica or not, preferring to go by tests and reports. Many Olympus m4/3 lenses for example are excellent too.