Is there any difference between the main SIM card and the secondary SIM card in the POCO X7?

There is no distinction between primary and secondary SIM cards on the POCO X7.
The distinction between primary and secondary SIM cards on a phone refers to the roles or functions that each SIM card serves when a device supports dual SIM functionality. In general, the distinction is mostly about default settings and usage preferences, but in most modern smartphones, you can configure both SIM cards to serve a variety of purposes.