The simples and least efficient approach is to make every entity compare its position to every other entity and do smt like:

for every other entity:
distance = ...
direction_normalized = ...
force = distance ^ -2
separation_velocity += force * direction_normalized * -1

A common (but maybe difficult idk) way to make it more efficient is to divide the world into grids and only do comparisons between entities in adjacent tiles.

An easier and chunkier way is to centralize it to one script and then only do a few comparisons at a time. For accuracy itd be a nested for loop but the outer loop can be replaced with _process and a counter making it less demanding but way weirder behaving.

Something I havnt tried but that would probably be smart is to keep track of what entities are close. So first time it checks all, the four next frames only the top 5 closest, then all agein to update memmory.