The micro SD slot in a large number of Android cell phones allows you to insert an external memory of this type and thus expand the internal or factory storage. Unfortunately, not all devices come with this slot . Some modern devices, such as the Samsung Galaxy S21, are abandoning this feature, so that in these devices you cannot put a micro SD as is normally done in many Android phones..
External adapters to read SD cards
The solution to read micro SD cards in devices that do not support this feature is quite easy and consists of using an adapter that connects to the phone's USB port on one side, and allows inserting a micro SD slot on the other side. As Android devices commonly support USB OTG technology, as is the case with the aforementioned Samsung Galaxy S21, they will recognize the SD card plugged into the adapter and allow you to view, copy, move or delete the files on it.
Electrand Micro USB OTG to USB 2.0 adapter available on Amazon.com for around $ 10..
In my case, since I already had an SD, SDHC, MMC, MS, M2 and TF / micro SD memory reader for my PC, I paired it with a USB to micro USB connector that I had lying around, as seen below , and also the cell phone recognized the micro SD card connected to said reader without problem.
In the case of iPhone it is the same. You can buy a similar adapter on Amazon for around the same price, like the one seen below..
SD card reader for iPhone available at Amazon.com
Even if your cell phone does not support the OTG connection, you could use as external storage a pendrive or wireless USB flash memory like this one from the Sandisk brand , which allows you to connect to it wirelessly.
Internal micro SD adapter
All of the above implies having to carry an adapter to read micro SD cards in a cell phone that does not support this feature. Another downside to this method is that you can't take advantage of the adoptable storage feature implemented since Android 6 , which basically means converting the SD card to internal memory . It is precisely for this reason that Reddit user "Sockpockets" made a crazy homemade modification a long time ago to put an SD slot on his Nexus 5 2013 mobile. Basically he connected a micro SD adapter to the micro USB connector of his device.
Apparently, the modification consisted of relocating the lower electronic board where the micro USB connector is located and ordering the speaker and antenna cables. Unfortunately there are major drawbacks with this hack: the back panel does not fit on the phone, and although it was replaced with an alternate panel, the latter left the camera unprotected and vulnerable to dust. Also, while this "internal USB OTG adapter" is connected, the device cannot be charged with the USB cable, so it is necessary to use wireless charging.
Replace the internal memory with a higher capacity one
Obviously a homemade mod like this is perhaps the craziest way to add an internal SD card reader to a device and have more storage. Replacing the internal memory with a higher capacity one perhaps makes more sense, both on its own and by third parties: apparently in China there are already factories that replace the memory or NAND chip of a 16 GB iPhone with one of 64 or 128 GB at a cost of between 70 and 90 dollars.
6 ways to increase the storage of a cell phone WITHOUT an SD card
Types of SD cards and which one to buy for your cell phone
How to know which micro SD my cell phone supports?