Question - If I buy a share over the weekend, lets keep it simple, lets say I buy 1 share and Trading212 £20 for the purchase of this share for when the stock market opens on Monday (NASDAQ). Subsequently this share rises 5% during premarket and opens at the higher price.
Well first things first. You cannot buy stocks over weekend.
You can place market/limit orders during weekend. But they would be executed when market is opened or price reached.
If you place market order based on share count, aka you want to purchase 1 stock of ABC, market price was 20£ on close at Friday.
On Market opening today it goes up 5%, the best available price will be around 21£, thus you would pay 21£ for this 1 stock.
If you put market order based on value, 20£. Instead of getting full 1 share at 20£, you would purchase fractional shares in value of 20£, effectively getting around 0.95 share count, due to full stock now priced at 21£.
Or if you put limit order at 20£, price goes to 21£ on open, thus limit order will not trigger and stock will not be purchased.