OEM stands for Original Equipment Manufacturer.
It does not mean a "BMW Genuine Part" but by required standards is the same or even better in quality.
During BMW or MINI vehicle production, BMW, like many other corporations, outsource their needs to various companies to produce parts for them during the assembly process.
BMW defines a mechanical design specification and quality standard requirement then sends a "request for bid" to numerous corporations.
These corporations submit their bids and, if won, are granted a production contract from BMW.
Examples: BMW shocks are not made by BMW but usually with Bilstein or Sachs. Your spark plugs are not made by BMW, they are made by NGK or Bosch.
BMW Brake pads are not made by BMW, they are made by Pagid or Jurid.
BMW didn’t make your alternator either, most likely Valeo or Bosch made it.
These companies are all OEM parts suppliers for BMW.