Thunderbolt and USB – C

No.

You can plug USB-C devices into your 2012 Mac Mini's USB 3.0 ports using USB 3.0 to USB-C adapter(s).

for example > https://www.amazon.com/USB-Female-Male-Adapter-Pack/dp/B083XXLW77?

If you need more USB 3.0 ports, then you can get and use a self-powered 4 port USB 3.0 hub.

for example > https://www.amazon.com/Sabrent-Individual-Switches-Included-HB-UMP3/dp/B00TPMEOYM?

While USB 3.0 is slower than USB-C, it is plenty fast enough for SATA HDDs and SSDs.

USB 3.0 to USB-C adapter(s)...

While USB 3.0 is slower than USB-C...

This is not technically correct. USB-C is a connector, not a standard. Not comparable with USB-3. The adapters you refer to are USB Type A to Type C. Their speed depends on what’s supported and what’s connected.

No.

Thunderbolt 1/2 Macs can only connect to Thunderbolt 3 devices

with a Thunderbolt3->Thunderbolt2 adapter.