That is not true. The IRS sends letters through U.S. mail when they propose a change to the return you filed, and whether there is a refund or additional balance due, they indicate what numbers differ and what penalties, if any, were applied.

You can then either accept the change, or disagree and reply with your position on the matter.

I've seen dozens of such letters over the years (both my own and others).

But I've never felt like I've had any clue why things were adjusted or what issues there were.

I'm probably way off base & things aren't actually so bad, but I feel like I get a lot of sympathetic "me too"'s about this in social circles. Maybe a good number of people are just holding their tongues & actually understand & learn from their errors over time, but I don't feel like my tax filing skills have ever improved.

If your taxes are more complex then expect more confusion. But at that point you should probably hire a CPA

Microsoft will hire some lawyers/accounts (or already has inhouse) to handle it.

All in all a scary but good experience.

Typically these very large orgs will also have dedicated relationship managers.