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The safest option is probably to remove the battery and run the phone from a (well defined/build) external power source, e.g. like this: https://yaky.dev/2022-09-06-smartphone-without-battery/

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Yes that's what I do too! I didn't know that blogpost but I generally replace the battery with a DC-DC converter module from Ali. You can even tune the voltage and see the indicator change the battery charge level. I put them around 4 volt. The advantage of these modules is also that you don't need a beefy 5V power supply, you can simply grab a 12V or so (I have a full drawer of those from shucking external hard drives) and it will regulate it down nicely. They are 80 cents a piece (10 for 8 euro): https://es.aliexpress.com/item/1005007508685885.html and they can deliver up to 3A or so which is enough to handle the peaks. They're a bit bulky but I just tend to mount them elsewhere. Make sure you use a fairly thick cable to the phone then to minimise losses at that low voltage.

I didn't have to bother with the thermistor but some models need it yes. And some even have an i2c charge level module which is even harder to simulate though it can be done.

By the way the diode option is a decent one too, I went for this option because I had many 12V PSUs around, and due to the efficiency of the converter I can use ones with pretty low amperage. But if you have a decent 5V supply using a simple diode drop is not a bad option.

What is bad is using a resistor as the voltage drop will depend on the current being drawn by the device, but the article is not doing that (nor woudl I).

Not all phones work without a battery in the case.

The link provided by cydodon is about emulating a battery.

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