Choose and ISO or IMG File, insert you SD Card or USB OTG into your phone, And write the Image to the SD Card/USB OTG.
NEXT STEP : Boot your Linux, FreeBSD, Windows or any other ISO on you computer.
** Needs root access. **
** Create your own distro **
Note that large SDCards/USB won't work due the limitations of FAT32 filesystems.
Make sure your SD Card or USB Stick is properly recognized by Android.
Maybe it will not work with these devices:
Samsung Galaxy S8+
Huawei nova lite
Xiaomi Mi 5 (gemini)
HTC One M9