Question about hooks

[omega1]

Hello, I'm new to WHMCS (late to the game!).

I have a question about hooks.

I need to create some tasks on my own server on order completion, no problem with the code to create the tasks on my server, just need to understand a little more about how to achieve this with hooks.

I need to run some code on a completed order (paid for) and need to grab a custom field (username) which is entered during the ordering which I have set up on the product and I also need to know which products the customer has ordered. I'd also like to grab the password they used on account creation so that I can use the same one on my platform (separate to WHMCS platform).

Ideally I'd like to do this in one hook file when they pay for the order and it is complete (rather than in multiple hooks).

Also, slightly separate to the above, I'd like to check the username they have entered doesn't already exist on my platform before the order completes, is there any way to do this?

Thanks in advance for any assistance!