This is not quite correct. It used to be. Originally the digital key for BMW required a particular model of Samsung phone. That then was replaced with only certain Apple phones. Now it will work on some Samsung, Pixel and Apple phones, but only if your vehicle was manufactured after a certain date. I understand BMW moved from digital key v1 to v2. Earlier models had the fearture disabled.
The Key for5 was the ability to sharethe key, so you could have up to five others with their own key to your vehicle. That always had to be purchased from memory. The basic digital key (using the phone) remained free.
In addition to the above they have a physical key card. That card is supplied with the vehicles in some locations (US for example), but has to be purchaed seperately in others (such as Australia). I obtained the key card when BMW disabled access via my Samsung phone.
To make it more complex, a phone based key could be added to a profile and obviously you could have more than one phone based key per vehicle. You can only hae one key card per vehicle though, and not add seperate key cards to each driver profile.
To say it is confusing is an understatement, and in the time it has taken me to post this reply, BMW has likely changed it all around again!
I am not sure if the second hand key cards will work. From what my dealer said, the key card had to be ordered specific for my vehicle using my VIN. It was then paired using the same process for the phone key. If that is true (or if it remains true) then 2nd hand key cards wouldn’t work. I suspect it i (or at least was 18 months ago) true, as I originally attempted to order a key card from a dealer on line in the US, and was advised they couldn’t send it to me as it had to be specific for my vehicle and had to go through my dealership.
But yes, an Apex solution would be simply wonderful.