Why is there a exclamation point in a circle when I start up my Mac?

The most common reason this happens is when your Mac is interrupted during an update, and it's a sign that your Mac's firmware is "broken". Here's Apple's explanation: If your Mac starts up to an exclamation point in a circle - Apple Support

I'm assuming your Mac is starting up looking like one of these screens? If your Mac doesn't start up all the way – Apple Support (UK)

Follow the link on the one with the exclamation mark to see how to correct this.

Hope this helps.

RESOLUTION: I'm assuming it's the network (https://www.quora.com/Why-do-I-see-a-yellow-exclamation-mark-inside-a-triangle-on-my-network-connection-icon) I've seen the exclamation point on the Wifi symbol *itself*, but never separate in a circle like this. Apple has created and magnified the "check engine light is on - what?" problem again. (Sarcastic-but-not-really) Do they put a "cheat sheet" in the box when you buy a new mac?

EDIT: This is a exclamation point in an orange circle in the menu bar when the mac is already booted, not while starting up.

What if it shows up right after waking the mac up, but goes away after about a second?

Background: The power is out, and I hadn't told it "connect to iPhone" yet. I had before, but it seems to ask each time after it's been asleep for a while - the iPhone probably shuts down data sharing after a while. Would this indicate it tried to connect to it's previous Wifi (the phone) and couldn't?

And I found this (https://ormsr.cosinecouture.com/triangle-exclamation-point-when-i-opened-mac/30569437) Whois reveals it's registered to "srsplus", which is a "web domain broker", so I assume some obscure search-engine-optimization (SEO) technique.