Different type of gasses

Would like to know briefly why cO2 is used for mig but argon is used for Tig and why you can't mix them.

It's not a simple MIG vs TIG question, unfortunately, but it's really down to the chemistry of the interactions of the base metal, the filler rod/wire, and the shielding gas mixture and how it affects the process. The particular type of the welding process, under the TIG/MIG main groups, can also be important.

For example, CO₂ should be fine for alloys with low reactivity, for example mild steel, but some metals are VERY reactive, for example magnesium and aluminium, and will "pull" the oxygen out of the CO₂ and oxidise the base metal and/or filler. These will require an inert gas, like argon.

However, it's not that simple, and because of the chemistry there are some rather exotic gas mixtures used for some specialist applications, for example - https://primeweld.com/blogs/news/types-of-gases-used-in-welding - and those first six are just the main ones.

Ah, a better guide - https://weldguru.com/welding-gas/