What triggers an HMRC audit?

I'm somewhat curious about the workings of the HMRC audit process. Could you enlighten me on what typically prompts an HMRC audit? Is it a random selection, or are there specific factors that increase the likelihood of being audited? For instance, does the size of one's tax return, the complexity of their financial affairs, or past audits play a role? Also, are there any common red flags that HMRC looks out for during an audit? I'm trying to gain a better understanding of the audit process and how it works to ensure my own tax affairs are in order.