Generally when you use a broker, they require to have the money...

Generally when you use a broker, they require to have the money available in your account prior to buying (Commsec is an exception, they will allow you to buy without funds in your account as long as you have the funds in on settlement date).

there's a lot to unpack when breaking this down but I'll try and keep it simple:

So even though it appears like the funds are gone from your account and your broker shows you with shares, you don't actually own them and they aren't paid for yet (this is where there is a lot more to unpack regarding settlement/clearing houses defaults etc). Most Brokers allow you to still trade unsettled securities because they remove the funds from your account and hold it for settlement date so it's ensured you meet settlement.

If shares go into trading halt, all outstanding settlements still take place, just no new sale contracts can be made. I also believe (not 100% so someone feel free to correct me), a trading halt means no securities can be traded at all. ISX has been delisted so it can be traded just not on the ASX.