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.)

Anyhew.
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:
instructables.com/id/Recycled-Cellphone-Battery-Charger/
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.