Now I don't understand taking the Fifth. If everyone takes the Fifth, how do you convict someone? Find evidences, and have the a grand jury find the person guilty?
Finding evidence and having a jury (not a grand jury, grand juries indict, but do not convict) find someone guilty is exactly how trials work in the United States. One of the fundamental tenants of the US legal system is that nobody is obligated to testify against themselves (except in some very limited circumstances). It's the job of either the government (in the case of a crime) or the opposing party (in the case of a civil action) to prove that a person did things they are accused of, not the job of the person to prove they did not.
The Fifth Amendment is often misunderstood. It only prevents the government from being able to compel you to confess to your own guilt. It does not prevent the government from during conclusions from other sources, nor even does it prevent the government from making you provide evidence to support its claims.
For example, say you habitually recorded all conversations in your office, and one day you talk about covering up a burglary you ordered. You cannot be compelled to reveal this fact: if the government asks you "Do you record your conversations?", you can plead the Fifth, and the government can't order you to turn over recordings of conversations you may or may not have. However, if you admit that you do (or someone else tells the government), then the government can order you to turn over recordings, and you cannot plead the Fifth.
You can plead the Fifth in a civil case if you believe that answering the question could subject you to criminal prosecution. But, unlike in a criminal case, where you have an absolute right to remain silent (and it can not be held against you), a judge or jury in a civil case can draw whatever conclusions they wish from your lack of testimony.
That and good old fashioned greed.
No matter what got you fired.
Think of it from the standpoint of a small-company CEO, on the margins, trying to get traction any way possible. Yes, 90% of them won't want to hire you -- at least on 90% of the days that they come to work. But inevitably, one of them will say: "Let's give it a try." They need brainpower. They need people who have played in the big leagues. And the notion of taking a risk on someone that their archrivals have discarded ... with at least the faint possibility of making it work ... is very hard to resist.
Yeah, things aren't looking awesome for him to work for anyone else. Perhaps he should look into starting his own company in the self driving car space. It worked out well last time he tried! /s