Read Where the logs are first and keep it open: finding the right log is most of the job, and the table there says which log answers which question and who can open it. The next page takes the one you will open most, the Template Errors screen, and walks one failing request from a vague report to its cause.
The failover page covers what a static page can do when the site cannot answer, and the notice an administrator can put up for planned work. The health checks page turns the quiet failures (a scheduled job that stopped, a certificate about to expire, a payment method back in Test Mode) into alerts with thresholds. The last page is the handover pack and training plan, which decides how the site is judged once the agency has moved on.