I’ve understood that electrons move in a 3D field outside the nucleus and that they are quite far away from the nucleus itself (in relation to the atom scale).
What would happen if you pushed them out of this moving field of theirs, closer to the nucleus?
In normal conditions, this is almost impossible. You can’t just build a wall around an electron to limit its movement, and there is an inherent uncertainty about its position anyway. A better explanation would require concepts from quantum mechanics, but it wouldn’t be an ELI5 anymore.
In very extreme conditions, like the collapsing core of a supernova explosion, electrons are squeezed really really hard between the nuclei. In this case they can enter a nucleus, fuse with a proton and turn it into a neutron. The entire core of the star becomes a ball of neutrons, also known as a neutron star. This is the densest object that can exist, short of collapsing even further into a black hole.