Our program is in RAM and takes some space there. Nevertheless, we still have a bunch of RAM. Imagine that it is all free, so if we need some place in the RAM, we can say to the operating system, for example, Windows: give us a little more space!

The operating system will give or not give additional memory if there is no free place, then you will have a problem :(

Well, now we are only considering the situation when everything is fine with the RAM, there is still a lot of free space, and you can safely beg for some memory.

In such situations, you will use dynamic memory: We do not know how much memory we need.