MiSTer adds several daughter boards to original DE10-nano board. You don't need to install all boards. MiSTer starts from entry level as a bare DE10-nano board. With just one additional board (SDRAM), you will be able to use almost all cores.

• SDRAM board (recommended expansion) This small board plugs into the GPIO0 connector of the DE10-nano board. Whilst the DE10-nano has fast DDR3 memory, it cannot be used to emulate a retro EDO DRAM due to a high latency and shared usage from ARM side. This SDR SDRAM on daughter board is required for most cores to emulate a retro memory module.
• I/O board (optional expansion) This board plugs into the GPIO1 connector of the DE10-nano board. It provides a legacy VGA output (6 bits per component), analog audio (3.5mm phone jack), digital optical audio, buttons, LEDs. This board is useful if you prefer VGA over HDMI or you want to put the MiSTer inside a case. This board also helps for core development (HDMI scaler code requires around twice more time to compile). Compiling for VGA-only will speed up the development. This board is not required to run most cores.
• RTC board (optional expansion) This board is plugged into LTC connector and provides a real time clock. You still can have a real time clock without the board if MiSTer is connected to internet via Ethernet. Only two cores use this feature, so it is only for enthusiasts.
• USB hub board (optional expansion) This board adds 7-port USB hub sitting under the main board.

Schematics and gerber files are available to download. Boards are considered as DIY(do it yourself). There are no restrictions who and how these board will be manufactured and sold, any 3rd party is welcome to manufacture and sell them.

https://github.com/MiSTer-devel/Main_MiSTer/wiki