It's a corporate culture thing. Most companies say nothing
I've never seen anyone straight-up fired except those who were terminated "at will" (I live in an at will state). "Hey Joe, today is your last day. Thank you." I've only seen the "at will" thing done once.
What we typically do is we "manage you out" or we "package you out." There is also "eliminate the position", but I've only seen once and the person got a package (money and a iron clad NDA/agreement) on the way out.
The other thing we do is just put people on a list for the next Reduction in Force (RIF). You can terminate anyone in a RIF, literally. So, when you have a RIF, you clean house.
Managing Out
With "managing you out", managers use subterfuge to basically crush someone. They will provide little direction, will document little in email, and will often contradict themselves to suit their needs. You'll get a luke warm mid-year review, followed by a Performance Improvement Plan (PIP) that you will never be able to "pass" despite whatever your company policy states, or to whom you escalate. You will get dinged on anything you can imagine: Internet browsing, coming in late, etc. The comments in the PIP will be extremely subjective and will be able to be swayed in your manager's direction. I don't care what you think your company would or would not do but the company is there to protect the company, not the employee. At the end of your PIP you'll get put on a 30 warning, and at the end of that, you'll be straight up terminated for cause. I've seen it too many times, especially in financial services.
Defenses against this. Take contemporaneous notes of every meeting you have, including the time, date, and who was present. Email this to yourself and/or print it out. If you live in a 1-party consent state (check your state laws), record everything. Your notes will be more believable than your manager's recollection of events.
Most people just bend over, but the smart ones consult with an employment attorney who will guide them through the process of collecting evidence. At some point, if you have enough evidence, your attorney will get involved and depending on how egregious your manager is, you'll either get packaged out (below), retained in your role, or terminated for cause.
An attorney for a company I previously worked for and who specialized in employee investigations / employee wrong doing once told me, "documentation is your friend." I did cyber security and interfaced with the person who I came to befriend. The advice was sage.
Packaging Out
This is the best. You get money, get put on "garden leave" (that is, you basically stay home and do nothing until your last day). The condition of your departure from the company will be that you resigned. But, you'll also sign an NDA/agreement that also releases the company from any and all liability from any and all current or future claims against them.
