Ken White has been talking about the potential for this suit for a couple weeks now. He spends a lot of time talking about what
isn't defamation --- people use defamation suits and threats thereof to chill free speech, and he's a 1st Amendment guy --- but the Dominion cases are apparently examples of very serious defamation cases.
The core of defamation is (1) a false statement of purported fact that (2) causes damage (there are some exceptions to (2) that aren't relevant here). Most defamation cases you hear about fall apart on (1) - they target non-falsifiable statements of opinion, which can't be defamation because they don't purport to relate an objective fact. What makes Dominion's cases terrifying is that they are chock full of purported facts, all of which are batshit, and have so devastated Dominion's reputation, in a reputation-intensive business, that it had to hire private security to protect its employees from death threats.
What makes the case challenging for Dominion is that they will almost certainly be treated as a public figure for the purposes of the case. The standard for defamation of a public figure is higher than that of a normal person; we add a condition (3): actual malice or negligence, meaning Dominion must show that Giuliani either intended to destroy Dominion's reputation by spreading facts he knew to be false, or that he was at least negligent, acting with reckless disregard for the truth.
The problem is that it's not totally clear whether Giuliani believed any of his crazy-talk. It may actually be the case that Giuliani is just off his rocker. If he himself was sold by the (false) facts he received from others, he is a more difficult defamation target.
Here's attorney Akiva Cohen analyzing the case a bit:
https://twitter.com/AkivaMCohen/status/1353700596407283721