Considering that one is a very novice swimmer, it's challenging to even do sporting activities in the water without a life jacket. I'm looking for ways one can build more resilience and stamina with breathing techniques as well in open waters.
The straightforward answer is that you train yourself for resilience and stamina by practicing.
The good thing is that strength, technique and stamina are all trainable in a pool, which is much safer if you are not a strong swimmer, and are not used to cold/open water. Before open water swimming you really do want to be a strong swimmer in safer waters.
Once you have your strength, technique and stamina at a level that will allow you to swim the distances you want, then start training in open water - ideally with a group, and using something like a tow float - these not only make you very visible in open water, but in an emergency you can hold them and stay afloat. You don't need to go into deep water, but initially swim parallel to the shore - you can even swim close enough that if you put your feet down you can stand up, but be carefull if you have obstacles, rocks, sharp surfaces that you may hit.
Cold water has a very different effect on your body than a warm pool - and this can catch people out, as their muscles can stop working in the cold, or breathing can be difficult, and it can make some people panic, so be careful!