Thank George, Andreu and Claeys!
in file manager of android, it shows "wlan direct connect"
note that UI for android is in Chinese, "direct connect" is my translation
i choose it, some machines are shown, but i can't see debian pc
i'v not been able to download termux
i can't install kde connect for android, google play is blocked in china
i can't use pairdrop
i have wished it can be done without installing additional software, but
file transfer over wifi isn't quite standardized as i hope
i give up. those big files in android aren't so important after all. for
small files i can use bluetooth
PS: i might attempt impossible: transfer 3G file at 50k/s via bluetooth,
which takes about 17 hours
PS2: i look into usb socket of phone, i see nothing that might obstruct
connection
