I don't think Tesla really hired any battery engineers or R&D either come to think of it.
If they did, they would have had an R&D office in Japan or SK who are leaders in Battery Chemistry but I don't think they have a presence there at all.
Most likely Tesla and Panasonic are co-sharing IP with Tesla taking a lead on Battery Management Software (a very hard problem) and Panasonic on Battery Manufacturing and Efficiency (an equally hard problem)
Panasonic now has another cell factory in North America, in DeSoto, Kansas. This time, Tesla is not the landlord.
A "cell" is a single unit device, and a "battery" is a group of connected cells. Panasonic makes cylindrical cells in standard dimensions. Others assemble the cells into into battery packs. Much of the rechargable battery industry works like that. Plants that make cells are mostly chemical plants, and some of the chemicals are corrosive, toxic, and flammable. Once the chemicals have been canned into a neat little metal can, assembly into packs is a routine manufacturing operation.
It's possible to make integrated batteries. Classic 12 volt lead-acid auto batteries have six cells, producing 2 volts each. BYD is heavily into integrated batteries, with their "blade" technology. There are lots of things that can go wrong in integrated battery manufacturing, but if the process can be debugged, the density is higher and the cost is probably lower.