The only place to buy Apple OEM refurbished devices is from Apple itself. Apple refurbished devices are like factory new devices, with OEM new battery and display. They also come with a full one year Apple warranty and are eligible for Apple’s extended warranty.
Many third party sellers offer their own refurbished devices, where the refurbishment is done by them or their contractors (e.g. Amazon refurb iphones are refurbished by Amazon or their contractor, not by Apple or Apple’s OEM contractor).
So check who actually refurbished the device, what does their refurbishment actually mean (note that new OEM parts are not available to anyone but Apple and their Authorized service providers), and what sort of warranty they offer (since it will have no warranty at all with Apple). Again, for example, an Amazon refurbished iPhone will have a warranty from Amazon, not from Apple.
Quite honestly, what some sellers put out as refurbished is nonsense. They’re just selling cosmetically cleaned used devices and nothing more.