I am trying to make Kerala style beef curry using a pressure cooker and I'm struglging to figure out how to make the beef go soft. To what I understand, the more we cook beef, the more harder it get, so how is it possible/ that I can get the beef in the curry soft and tender as it is when served by local shops?
Tough cuts of beef that are typically used in stews have lots of collagen, which is what makes them so tough. Collagen turns into gelatin when exposed to heat (and moisture) for a sufficiently long time. Typically this means cooking it for several hours on a low heat. Gelatin is liquid at warm temperatures and it gives meats and the sauces they are cooked in a silky, rich texture. I'm not sure if pressure-cooking would break down collagen in quite the same way, or do so without also drying out the meat (as higher temps will force the water out of the meat fibers, even if they are bathed in liquid).
Naturally tender cuts like steaks have almost no collagen. With these cuts, it's all about internal temperature. A high internal temperature results in dry, gray meat (especially for lean cuts that also have very little fat to keep them moist). To keep these cuts tender you need to stop cooking them when they reach (at most) a medium doneness, which corresponds to a core temperature of around 60°C. This is way below typical simmering temperatures for something like a curry, which will be upwards of 80°C. So with something like a steak, you'd typically want cook the steak separately (e.g. in a skillet or on a grill) and then add it to the sauce at the last minute (making sure the sauce has cooled down some). Or even keep it separate from the sauce altogether, or serve it on top.
For an Indian curry I think you'd typically use the first type of beef: a cheaper, collagen-rich cut (something like chuck) that benefits from low-and-slow cooking (braising or stewing). If you're not getting good results with a pressure cooker I would try a normal pot instead (or a slow cooker if you have one) and keep it at a bare simmer until the meat is as tender as you want (which you can check periodically by poking with a paring knife).
Beef has many cuts, some are inherently tender, like filet, sirloin and ribeye. Cooking those too long dries them out, and you don't want to do that.
Working cuts like shin/shank, neck, flank, brisket and round on the other hand are inherently tough as they are full of collagen, which is a type of tissue which distributes force throughout the muscle. Collagen breaks down into gelatin with heat and moisture, this can be done low and slow or faster in a pressure cooker.
It is not really correct that meat gets harder the more it is cooked, for most meat there is a "toughness curve", where cooking for either a short period or a long period produces tender meat, but anything in between can result in tougher meat.
Some cuts and types of meat are more forgiving (in general fattier cuts are a safer bet for a range of cooking approaches), while others lend themselves to shorter or longer cooking. Very lean cuts tend to be better with less cooking otherwise they can be dry, while cuts with lots of collagen tend to be better slow cooked (although this isn't strictly the case in all dishes, for instance if lean meat is stewed for a long time until it falls apart it will be moist from becoming blended with the sauce it is cooked in).
In a pressure cooker, provided it is cooked for long enough, you shouldn't have a problem with meat turning out too "hard". I have cooked beef curry in my Instant Pot, which I cooked under pressure for 35 minutes. You may need to experiment a little with timings to get the exact result you want, and the perfect length of time will partly depend on the cut of meat you are using, but I'd suggest cooking for at least 30 minutes to be on the safe side.
Meats also usually turn out more tender if you leave the pressure to release naturally, rather than manually releasing the pressure quickly. I'm not very familiar with stove top pressure cooking, only the Instant Pot, so I'm not sure if the process is exactly the same, but with the Instant Pot you just wait until the valve in the lid drops before opening it for a "natural release".