The Boya DM-20 is a tiny mixer with two 3.5mm inputs. The maximum gain is 30dB but it can only be powered externally via USB port.
Two volume dials for independant control of each channel with mono or stereo selector. Unlike passive mixers, BY-DM20 can supply mic bias to batteryless shotguns such as Rode micro/micGO.
Two outputs are included. One digital via included USB-C cable and one monitor level output.
This is a great choice for smartphone videography.
It can also be a convenient mixer for small pocket cameras like Gopros, Canon G7X mk3 or the RX100 mark 7 if running off powerbanks/battery handles (although an attenuator cable is probably needed for the headphone out.)