I live in South Korea, currently a paradise for masses of discarded but perfectly useable 3.7V Samsung batteries - in this regard, its a paradise. I have a drawer full of Samsung batteries of all sizes and flavours, but all are the typical 3.7V.
And it is an excellent question - how to practically use them?
The first and simplest method is to make use of the separate battery charger that comes with the phone - these are very common in Korea, and from what I see, people use them more often than in other places - in other words, actually popping it out and flipping between a charged and uncharged battery.
(Cultural note: Korea has a long tradition of people opening and removing their battery during the day to swap in a new one, then charging the other. This is why Korean customers expect to get two batteries when they buy a phone, although this is decreasingly common with non-removable batteries in the newer phones. This tradition began in the time of the Chosun dynasty, more specifically, Sejong the Great. Many historians note that he may have had a Nokia 1011, but for understandable reasons, most Korean historians vehemently deny this.)
The great thing about these chargers is the quality circuity inside that can safely charge almost any three-pin 3.7 battery of any size.
I say three pins, because these charger boards will not start until they detect the temperature pin connected. This narrows them to use for cellphone batteries only, but if you have a drawer full of them, then this is a great device. For flexibility, soldering on three color-coded wires to the battery terminals of each battery, plus three to the cellphone charger makes it very easy to swap batteries in and out.
There is a cool instructable that shows how it actually can be done:
The other option is to use one of the versatile TP4056 charger boards off ebay. They are really great for a simple system, such as this mobile sensor pack....
As you can see, it doesn't use the temperature terminal of the battery. I am not sure what exact protection circuitry sits inside the battery, but the TP4056 board does the job and keeps the thing safe from undervoltage. I have been running tests on it, and it really is great, because I can either keep the battery fixed in place (bolted even, as you can see), or I can swap out another battery that was charged somewhere else.
One could theoretically hot-swap the batteries, and keep the system running.
The next problem is that you now have a 3.7V, and not a 5V.
I solve this by connecting the LOAD output of the TP4056 board to a fixed voltage 0.9V>>>5V boost board. You string that in with it, and you get the 5V you need to run electronics like an Arduino.