It works whatever way is agreed upon between them and the investors. For such large amounts it’s unlikely to be pure cash (there’s likely some amount of services somewhere in there), and they won’t be calling for all that cash at once.

The cash that is guaranteed is sent as soon as the investee needs it (they do what is called a capital call). Early stage startups and investments just do one capital call for the full amount, but larger amounts are often committed for periods of time; this also helps the investors schedule their own cash flow: for example if I have 500m this year and 500m next year, I can invest 1b in you, given the right schedule.