If we want to attempt understanding this at the atomic-electron level, imagine this illustration. You have two sources; one which wants to output 10 electrons / second and another which outputs 20 electrons / second. You connect these with a 10Ω resistor. Because there is 10 electron/sec difference in "voltage", coupled by a 10Ω resistor, the current that flows is 1A. But this current flows from the bigger supply, through the resistor, into the smaller supply - even though it is a "supply."
So yes, energy is being forced into the 10V supply from the 20V supply, even though they both are intended to be outputs. Such effect might be called "back-feeding" and generally is not a good idea.
What happens to that energy, where does it go? Well, it almost certainly is dissipated in the 10V power supply as heat.
There are many different topologies of power supply, and each has various strengths and weaknesses. In general through, inside both supplies, components regulate the voltage and current output to safe limits. Depending on how robustly the supply was engineered, this safety margin may be small ("cheap" supplies), or larger ("expensive" supplies.)
- A supply you find on eBay, Amazon, Aliexpress for $10 is a "cheap" supply. These are typically designed on the low-side of the robustness spectrum. They can work fine for their intended purpose, but are not very tolerant of abuse such as back-feeding. This type of supply has every conceivable cost-cutting measure taken, such as reducing or eliminating heat-sinks. Without a suitable heat-sink to dissipate extra power introduced by back-feeding, such a supply could fail quickly.
- An "expensive" supply is generally much more costly. These are more expensive because a) the additional engineering work done to make them more robust, and b) the addition of extra features/components such as larger heat-sinks, physically bigger components (to allow for better heat-sinking), thermal monitoring, etc.
In short, a more expensive supply is generally able to handle back-feeding better than a cheap supply. This usually isn't much of a problem, because system designers understand that back-feeding a supply should be avoided.
Incidentally, this is one reason why bench power supplies for electronics engineers are expensive - they are designed to be robust against several types of transient and continuous events, including accidental back-feeding.