The 7x15 backpack makes it really easy to add a 105-LED display
The LEDs themselves do not connect to the Feather. Instead, a matrix driver chip (IS31FL3731) does the multiplexing for you. The Feather simply sends i2c commands to the chip to tell it what LEDs to light up & how bright, and it is handled for you. This takes a lot of the work and pin-requirements off the Feather. Since it uses only I2C for control, it works with any Feather and can share the I2C pins for other sensors or displays.
The LED matrix uses only the 3V (second from left) and GND (fourth from left) pins for power and logic. Current draw depends on how many LEDs are lit but you can approximate it as about 40mA for most uses.
Note that the 3.3V power supply is a tiny bit lower than the forward voltage for the pure green, blue and white LED matrices but we didn't find any significant degredation in brightness. Really, they're still very bright.
All LED control is done over I2C using the IS31FL3731 interface library. This means SDA (rightmost) and SCL (to the left of SDA) must be connected, see above for those pins.
The default address is 0x74 but you can change the address to 0x77 by bridging solder onto the address jumper