This varies between operators but, in general, first class tickets include more legroom, your own table, reclining seats and overhead reading lights. Some of them do offer free snacks and drinks, but this may be curtailed during the Covid-19 pandemic.
Some trains include complimentary Wi-Fi in first class.
If you have a first class pass, you may travel in first AND standard class carriages. If you have a standard class pass, you may only travel in standard class. Note that some trains do not have first class carriages.