Let us assume that the bid price is $10 the ask price is $20. We can only buy for $20 and only sell for $10.
Does the current price have to be in the middle? IOW, does it have to be $15? Or can it be $14 or $16 as well?
Who sets bid and ask prices? Do market makers still exist or is everything driven by orders now?
Can the spread change but the price be unaffected? For example, can the current price be $15 as before, the ask changes to $25, and the bid drops below $10? Or is that very unlikely?
During trading hours there are three numbers (the bid, ask and last price). The NBBO quote (USA) will always be the highest bid and the lowest ask.
The last price quote is always the price of the last trade.
In the absence of aggressive order placement, the market maker sets the price. While it's a bit extreme, let's go with $10 x $20 If buying, you are free to bid $10.01 for the stock and the quote then becomes $10.01 x $20.00.
You are now making the market on the bid side, at least until someone comes in with a higher buy price. The NBBO will fluctuate according to orders placed. If I placed a priced buy order like $19.50, the quote would become $19.50 x $20.00.