There's a few things here.

The government doesn't actually know who's illegal or legal unless they specifically check a physical person. It's not like they maintain a list of "illegal" people. Your name gets recorded when you enter the country legally, but it's not recorded when you leave. If you fail to leave, they don't really know until they find you and match you to the entry. If you entered illegally, there's no record at all.

Second, You could easily use a fake ID or fake identity to get a cellphone and the carriers wouldn't give a shit as long as the bill gets paid. It doesn't even have to be under your name, maybe it's under your friend's account.

Third, I'm not sure how prevalent this is, but you don't need a "cell" phone to have a phone. A lot of poor people just have a device that can connect to WIFI, and make calls through an app or just message.

I get what you are saying, but there is a pretty big difference between "undocumented immigrant" and "the authorities are actively looking for you." Also, it's pretty easy to enter alias information into a phone so you can use it without announcing that it belongs to you.

You can buy a prepay phone at Walmart or similar, then just buy cards to add airtime. You don't have to register your name anywhere. I had one like that for years.

They might know where a phone is, but not if it belongs to a 'legal' person or not.

Accessing that location data isn't trivial. The data is typically held by various private companies who put up at least token legal resistance to cover themselves from lawsuits.
Intelligence agencies have their own avenue for getting the data, and on paper they're not allowed to share it with police agencies.
Police agencies typically need to specify the individual in question, or the specific location and time to get a warrant. This is because they're not supposed to be able to blanket surveil an otherwise private piece of information without having a good reason.
The classic example is not being able to listen to every call on a payphone they know drug dealers use because they'll listen to people who have not done anything illegal.
Intelligence agencies are an entirely different thing with weird special rules and minimal and strange oversight.

This is all relevant because the government doesn't actually know who's allowed to be here or not.
Most people in the country without proper documentation entered legally and then just stayed outside the terms of their entry. The terms can be difficult to verify remotely, which is why you're not actually here illegally until you go in front of a judge, they deport you, and then you return again.

Finally, there are significant chunks of the country where location tracking via cell tower is imprecise enough to get the country wrong, and a lot of people live there. So any dragnet surveillance setup is going to have to exclude some pretty large population centers to avoid constantly investigating people in Windsor sometimes quickly teleporting into Detroit.

Law enforcement got their hands on the location records of the people who stormed the white house by... buying it from google.

Not every government has as much spying going on as some might think.

I assume they roll the dice because it's rather hard to get by without a phone.

Also, it's not like the government is actively tracking everyone's location. I'm sure if they wanted to track me they could, but it's not like my position is being actively logged right now.

Unless you're a person of great interest that level of effort just isn't happening at the scale needed.

They got better things to do than go after all the people just trying to vibe with the locals.

How would they know who's phone to track if the individual is undocumented?

They don't use tracking capabilities to help people, they use it for law enforcement. And if you want to track someone illegally entering the country cellphone triangulation is the least effective method. The person could: - Not have a phone - turn the phone off - use a burner phone and number. And by the time you've found them they're already across. If their traveling by boat they could drop the phone in the water aswell. And cell tower triangulation is inaccurate anyway, a 5G triangulation can pin someone down to the cm. (Half a hotdog for the Americans.) And you need to have towers close enough to the entry point aswell. If the tower is too far away it will be inaccurate.

In functional democracies, there is law protecting individual against the government. Meaning that you'll require a court-order to request the localization of a phone (to the phone provider or applications collecting GPS data). This is (in democratic countries) allowed in criminal matters but not for administrative status matters like immigration.

What the government is capable of doing, and what the government will actually do, are two vastly different things. Movies depict the government as this insanely competent, unstoppable machine, with endless resources and desire. The reality is that detectives won't even interview eye witnesses, since they'd have to leave their desk to do so. So, if a detective won't even interview people with critical information about a case, how determined do you think they are about tracking down someone who may or may not exist, who they have no knowledge of, who probably hasn't committed any crimes except for being here?

The word "theoretically" is doing a LOT of work here.

The big thing is whether people are behaving in a manner that brings them to the attention of the government.

It’s not like you have to give your SSN to a carrier to get a phone; the government needs a reason to be tracking you.

Now, they very much could put in a warrant for all phones crossing the border at unusual times/locations. But someone who snuck in with family and is working cash-only jobs to get by is unlikely to get tagged by the government unless they’re going somewhere or doing something the government is already watching.

Spending the resources and time to location track and arrest then deport people in such a way would be way too expensive and a waste of time probably

I've often thought of a method to evade tracking.

You create a group where you all share one group of phones with standard apps. You use one phone for a week, then place it into the group pool and select a different phone. You just keep reshuffling the phones over and over again. And even after a month or two, transport a batch of phones across the country to a different group for the same number of phones and just keep rotating phones everywhere all the time.

This depends on your (privacy) laws. The phone operators (and in all fairness, a lot of app developers) have access to the phone's location.

If the government doesn't need a warrant, and they can just ask for the location information at Apple or Google (or, for example Meta or X if those apps have location tracking permissions turned on), they could in theory find anyone they like at any time.