Mount an Android device on your pc filesystem over ssh

Install Termux and setup openssh server

Install Termux from Github (or F-Droid) on the phone you want to SSH into. Don't use the Play Store version, it doesn't work
run pkg update and pkg upgrade
run passwd and set up a password
run pkg install openssh
run ssh-keygen -A
run termux-setup-storage - this will make the app ask for storage permission
run sshd
run ifconfig to know you ip address - look at inet field that looks like 192.168.X.Y

Test ssh connection

port=8022
android_ip=192.168.X.Y
ssh $android_ip -p $port

The last command will prompt to digit the password you have previously entered. Usually ssh specifies a username (username@$server_ip), if your ssh client doesn't allow you to not enter a username, just use a blank space or asterisk.

Once connected, you can display your device internal directory tree as follows:

cd /storage/emulated/0
ls

Mount android with sshfs

android_dir=/storage/emulated/0
mount_point=/mnt/android-device

id $(whoami)

sudo mkdir $mount_point
sudo chown $(whoami):$(whoami) $mount_point

sshfs \
  -o idmap=$(whoami) \
  -p $port \
  $android_ip:$android_dir \
  $mount_point

If you need to troubleshoot sshfs connection you can add the following options:

-o debug,sshfs_debug,loglevel=debug

If you want to specify a custom ssh config:

-F /path/to/.ssh/config

Stop ssh server

pkill sshd

Unmount directory

fusermount -u $mount_point
sudo umount $mount_point
sudo rmdir $mount_point