@naofeifei Hello and Welcome.

As you have noticed, Ubuntu Touch (UT) is set up in a different way than Ubuntu on the desktop. UT has a read-only filesystem, similar to SteamOS, Fedora Silver Blue or Ubuntu Core. Because of this apt is not supported. Updates are provided via the system settings. Additional applications are installed via the Open Store.

Apps for UT are bundled as click packages, or clicks for short. The regular way of developing apps for UT is on a(n Ubuntu) Linux desktop with the Clickable tool.

But if for whatever reason you want to develop on your UT device, you will need set up your development environment in a container of some sort.

You can try doing this in a Libertine container. There have been sucesses going this route in the past.

If your device supports it, you could also enable snap support with the Snapz0r app. Then you could install the LXD snap and set up your development environment in a LXD vm or container.