RAM Type: PC3-12800 DDR3L Min. RAM Speed: 1600 MHz
Details: Supports 1600 MHz PC3-12800 DDR3L SDRAM SO-DIMMs (204-pin).
Standard RAM: 4 GB Maximum RAM: 16 GB*
Details: 4 GB of RAM is installed as two 2 GB modules, no slots free.
*Apple officially supports a maximum of 8 GB of RAM, but third-parties have determined that this model actually is capable of using up to 16 GB of RAM with two 8 GB memory modules.
No, they don't. They use replaceable modules.
This machine supports 8GB per Apple, which is done with 2x 4GB DDR3 modules. It may even take 16GB, but it's a crapshoot if it will work. The Intel core i series put the memory controller in the CPU, so the max RAM is CPU dependent these days. Look up what the max RAM for this in the Intel ark pages.
Apple will sell it with no more the 8GB max, but it can probably go to 16GB. See the link to Crucial below.