At the moment Apple does not manufacture or produce battery for iPhones and All MacBook Series. According to arstechnica.com in Jan 2019, Apple might have plans to produce it’s own batteries for iPhone and Macs in the future.
Apple engage several 3rd party manufacturer to produce battery for them which includes SMP, Simplo, DSY and more. There are mixed reviews online for battery quality on each manufacturer. Base on my personal experience and being in the IT servicing business for over a decade, there are alot of factors to determine the life span of the battery. We will have a post on how to pro-long the life span of your battery.
Just remember Apple only design and create the technology / blueprint, they engage others to produce for them.