How I can I increase my phone's memory or make it dependent on the sd card?

[AC Question]

My phone memory is very low and doesn't allow me to download apps although the sd card has space

 

[Rukbat]

You'd have to mount the SD card as the phone's main memory. That's done by /etc/vold.fstab. You'll have to learn enough Linux to modify that file. (It's going to be different for each phone.)