connect and pair device, get $MAC
create .asoundrc:
"""
        pcm.btheadset {
        type plug
        slave.pcm {
                type bluealsa
                device "$MAC"
                profile "a2dp"
        }
        hint {
                show on
                description "BTHeadset"
        }
}
"""
restart bluetooth and bluez-alsa services, then:

alsamixer -D bluealsa # set volume
mpv --audio-device=alsa/bluealsa (...)