Say there is a mass inside a liquid. What causes the force on its upside downwards and the force on its downside upwards? It seems logical that the downward force is due to the weight of the fluid above it, is it similar with the upward force? Just collision of liquid particles?
Consider a vessel of any shape.
At any 2 or more points is the vessel that are coplanar, no matter what the shape of the vessel, the pressure will be equal.
Just collision of liquid particles?
If an object is immersed in the liquid, pressure will act on all directions on it from the liquid's particles.
The net pressure (the pressure acting on both sides of the object cancels each other) acting on the on an object at a depth $d$ in the liquid is given by $$P=d\rho g+P_0$$ where $P_0$ is the atmospheric pressure acting on the free surface and $\rho$ is the density of the liquid.
is it similar with the upward force?
There will be an upwards pressure from the liquid's particles due to the buoyant force too.