Is it possible to use the iPad as a display for a Mac Mini through a wired connection only (USB-C to HDMI)? I am particularly interested in not using additional hardware nor third-party software.
No, the Mac mini is a desktop computer that requires an external display, keyboard and mouse.
The Mac mini needs to have an external display, keyboard & mouse, to setup and operate.
Then you can connect and use the iPad as a second display.
see > Use your iPad as a second display for your Mac - Apple Support
Can the iPad be the primary display?
Using an iPad as a display for a Mac Mini through a wired connection only
