What determines or how is the value of a share determined at any given time of trading? Is there a certain amount of transaction up or down before the value changes in that direction? Is it changed automatically or does someone manually do it? How often is it updated?
It’s based on the last sale
so if some is prepared to buy or sell at a certain price it will change price
As I understand it, the price is always a “historic” price in a sense - it records the price of the last sale but there is no guarantee that just because Megacorp is quoted at $3.75 a share, you will actually be able to buy (or sell) shares for that price. It all depends on who is currently offering or bidding for shares.
Stock prices work a little like house prices.
When you prepare to sell a house, your real estate agent looks at comps–comparable homes in the area that have sold recently–and looks at those prices, and figures your house should sell at about the same price. Someone makes an offer, there is negotiation, then your house sells at some price. Then your house becomes part of the comps when another house goes on sale.
The main difference is that there ways to evaluate the price of a stock without looking only at recent sales, like looking at the company fundamentals, etc. But the predominant mentality is to look at the last sale as a baseline for determining at what price you are willing to sell or buy.