gsmunlocker You can find "OEM Unlock" Button in UFI Android ToolBox in Fastboot Tab.
Thats will unlock bootloader, as long as phone support standart unlock procedure. But most newer devices use their own, mostly server base unlock, ex: xiaomi, sony, htc, etc. So this button will not work for such devices.
You can find specific unlock bootloader image for specific devices in gsm forum, for qualcomm mostly use modified aboot image.