Something I’m trying to understand is if I define a task;

30 mins estimated

10 workers

External Survey and matching the completion code from Prolific to survey end code.

How does it actually work in terms of showing the task to people? Does the system show to everyone and then the first 10 who accept the task work on it. After 30 mins, if they haven’t responded opens to more people or it shows to everyone and whoever submits first gets in?

You have it almost exactly right but it will take longer than 30 minutes before the system decides that any one of the first 10 participants have given up.

30 minutes is deemed to be your estimate of the median time, but the maximum time allowed is a based on a formula:

If one of those first ten goes over this maximum time then their submission will be automatically returned and one place will be made available to the rest of the participants on Prolific.