Audits? A letter from the IRS is generally not an audit. Identity Verification request is not an Audit.

So audit is just crunching numbers and a review is missing papers?

Basically, yes.  And an audit can require a personal visit, documentation of every income and expense item, copies of vendor invoices, etc.  Especially if you are a cash business.

Any audits from 2018 won't happen for at least a year.  An audit is a request to the taxpayer to prove their deductions or income, sometime via mail, sometimes via in person interview.

There are lots of other things that aren't audits.  Some taxpayers may need to verify their identity before getting a refund paid.  This is an anti-fraud measure, and is not an audit.  We don't know what the triggers are, but they may include things like, a customer who always does direct deposit now getting a large debit card; or a customer whose address has changed getting a large check; or many large refunds going to the same address.  (And you could still get audited later if the IRS wants to verify your income or deductions.)

The IRS also tries to match your income to reports from the payers.  Your employer is supposed to send a copy of your W-2 to the IRS.  If you claim a large refund based on a W-2, and the IRS does not have the employer copy, or it doesn't match, the IRS may delay your refund so they can verify your employment.  And they always try to match dependents, and won't allow the same dependent to be claimed more than once.