iPad 9 gen connect to mac mini

yeah

you can use a cable with the connector that the ipad has and a connector that the m4 has in the other end

Cables - Charging Essentials - iPad Accessories - Apple

or use adaptors to switch between usb-a, usb-c and or ligthining bolt

Amazon.de : amazon adapter usb c

According to the Apple Support article, the M4 Mac mini and the iPad (9th generation) are new enough to support Sidecar. Keep in mind that when you "work mobile", you will need at least one real hardware display (monitor, TV, projector). Otherwise you will have no way to see the user interface to connect the iPad as a Sidecar display.

Yes you can do that using the supplied charge & sync cable that came with the iPhone.

see > Sync content between your Mac and iPhone, iPad, or iPod touch over Wi-Fi - Apple Support

if it's a new enough ipad

read

Use an iPad as a second display for a Mac - Apple Support

Apart from syncing can I work mobile and use the ipad as a screen with the Mac mini ?