Buying and selling in the stock market I am a total beginner to this, but from the few online videos about how (practical) trading works, I understand that when buyers dominate the market the prices go up and when sellers dominate the prices go down (of course this is an oversimplification). I am not sure I understand how can buyers or sellers dominate. I thought then whenever someone sells some shares someone else buys them. If that is not the case, then where are these shares going, and where are the money going to the seller come from? Going to the extreme case of assuming that everyone sells their stocks, and no one buys, who owns the share anymore and who is paying the people who sold the stocks (which for a reasonable company they are worth a few billions)? Thank you!
I understand that when buyers dominate the market the prices go up and when sellers dominate the prices go down (of course this is an oversimplification).

People want to treat trading as some mechanical force. People are buying and selling shares, and the prices reflect where they want to transact. As news comes in, people can change their minds about a company’s value without any transactions taking place.
For example, if a company announces accounting irregularities and an imminent bankruptcy, everybody is going to revise down what they think the shares are worth. Prices will be adjusted lower by everyone, and you could even have more people lining up to buy on the theory that the company might have more value than what is suggested by the price.

I thought then whenever someone sells some shares someone else buys them.

This is always the case, by definition. To deal with you other questions, in an ideal world, if we add up all investor’s shares, the net position is 100% of the amount outstanding.
(The real world is slightly messier. Stock trading does not settle instantaneously, settlement is later. This allows short sellers to “sell” shares that they do not currently own - they borrow them from somebody else before the settlement. This obviously complicates things, but it makes sense once we realise these are credit transactions. A seller is not immediately selling, instead they have entered into a contractual obligation to deliver shares at a future date.)