Cellphones can be tracked even without a SIM card. Cellphones will connect to the cell tower that provides the best signal, usually this is the closest tower. Cellphones however will still contact other towers in order to estimate their signal strength. Whenever a cellphone contacts a tower, the tower takes note of the time, device, and signal strength. By using this data from multiple towers, it is trivial to pinpoint the location of a cellphone using simple maths. A computer can automate this and perform it in less than a second, which may allow real time location tracking.
This tracking is even more accurate in urban environments where 5G compatible towers are common. Because 5G has a lower range, there has to be more towers and these towers have to be closer together, which enables more accurate tracking. Not having a 5G cellphone won't help you here because most of those towers are also 4G/LTE compatible, even if they weren’t your phone would still contact them to get information about the tower. It's not the type of signal, it's the amount of towers and how close they are together that can determine how accurate location tracking can be.
When trying to defeat this kind of tracking removing the SIM card doesn’t work, the phone will still contact the cell towers for emergency calls. Air-plane mode is unreliable in a lot of phones, unless your phone is running Graphene OS or an alternative focused on privacy, you shouldn’t trust air-plane mode. If your phone has a removable battery, remove it, a phone with no power has no way of operating. Turning your phone off may also work but has proven to be unreliable with newer smartphones, the problem is how do you know the phone is actually off.
Possibly the only reliable solution for modern smartphones is to physically isolate the smartphone from the outside. Faraday bags, pouches, and cages provide the required isolation to prevent the phone from communicating with anything outside, this includes cell towers. Foil is a cheap alternative but depending on the phone it can take 5 all the way up to 20 layers to block all signal. If you do decide to buy a Faraday bag or pouch for your cellphone, be careful of which one you buy. Take note of how the bag closes, folding bags tend to wear out their inner signal blocking layer after a month or two of use.
If you take a laptop with you and connect to free Wi-Fi, it is possible to track your movements by tracking which Wi-Fi networks you connect to. Most free Wi-Fi will log the MAC address of your computer, this MAC address is usually persistent across Wi-Fi networks. By using data from Wi-Fi networks, it's possible to track a device’s movement by knowing what Wi-Fi networks the device has connected to. This is way less effective and accurate compared to cell phone location tracking, but it certainly something to consider.
In order to prevent this form of tracking, the MAC address of the device should be randomized. In addition, if the Wi-Fi network has a captive portal, enter different information upon every connection if the captive portal requires any information. Some operating systems like Tails OS randomize the MAC of the device by default when in use. Most Linux distributions also allow some form of MAC randomization through the usage of the network manager or through the macchanger package. If you opt to use the macchanger package, note that it sometimes doesn’t work, verify the MAC has been changed before connecting to a Wi-Fi network.
Some devices geared towards privacy come with hardware switches that can disconnect things such as the microphone, camera, and wireless radios (WiFi, cell towers, and Bluetooth). These hardware switches are as effective as physically disconnecting the microphone, camera, or wireless radio. These switches are also much simpler to use and may be more reliable than a faraday bag.