Can you do the beer brewing in a big 20ltr cooking pot instead of the plastic drums that come with the brewing kit
Yep! You can ferment in a cooking pot if you wish, but keep reading. The best fermentation vessels (like what pro brewers use) are indeed steel. Plastic buckets are more susceptible to getting a scratch in them and harboring bacteria and letting light in which can damage a beer over time. Glass carboys are heavy but won't scratch but let light in (so you cover it with a towel), can be a little tough to clean. Steel conical fermentation vessels are considered the best, but are expensive and hard to move around.
So why doesn't everybody just us a big pot to ferment instead glass carboys/brew buckets? Well, it is going to be hard to seal it up and have a one way air lock so the off gas from fermenting can be released. Once the wort is in the vessel you can't let any air get in there, otherwise bacteria can wreck your beer and/or have oxidation problems. Brew buckets (pretty cheap) have a sealable lid with a little hole in the top what you can put a $1 air lock. Glass carboys have a very small opening at the top where you put the air lock on. So, if I were you I'd hit up my local homebrew store and get a bucket, should only be a few bucks for that and an air lock. Later you can always upgrade to a glass carboy :)
On a side note you can ferment in a keg, just a wicked hassle to clean. Good luck!
If your pot has a snug fitting lid, then you'll probably be able to just use that.
I've been brewing in the usual plastic fermenters with airlocks fitted into a grommet in a hole in the lid. However after the first couple of uses, the seal on the lid breaks and it's rare to see any bubbles coming out of the airlock because the gas is escaping through the gap between the lid and the bucket. Recent I bought a bigger fermenter and it didn't have a hole pre-drilled in the top, so I just used as is, with the lid not quite pushed down all the way round, and don't think it had any ill effects.
The yeast will produce a very large volume of CO2 in the first couple of days, which will prevent air from getting in assuming it's not subject to a lot of turbulence, and if the opening to vent is small, and you're not moving the vessel around a lot, this should not be a problem. You may not want to leave your beer like this for a long time (e.g lagering) because eventually, however small the opening, the CO2 will probably mix with air.