Good day to you!
Almost all the modern Android smartphones and tablets support USB connection to external devices, such as flash drives, card readers, mice and keyboards. This doesn’t require any special drivers; Android supports such devices by default. The right way to connect them is described in my article below.
Connecting a flash drive, a mouse and a keyboard to a smartphone
Getting ready
To connect USB devices to a smartphone, we will need:
1. Android smartphone or tablet, supporting USB Host (OTG). Almost all the modern Android devices support this function, but to be sure that particularly your phone supports it, you can type in Yandex or Google “you phone model USB Host OTG”. If you’ve faced any problems with finding out whether your device supports USB Host – please, leave a comment with your phone model. We will quickly help you.
2. OTG cable, it looks like this:

This cable costs 0.5-2$ (30-120 Rubles), it can be bought from AliExpress.
If your phone is equipped with a common MicroUSB connector, then you can buy such a cable from these trusted sellers: Seller 1 (price: 0.46$), Seller 2 (price: 1.65$, the cable is nice looking and reliable).
If your phone has a Type-C connector, you can buy a suitable cable here: Seller 1 (price 0.54$), Seller 2 (price: 1.24$, the cable is nice looking and reliable).
If you have never bought from – consider my article: How to buy from Aliexpress (step-by-step instructions).
Connecting a mouse to a phone
All you need to do is just to plug an OTG cable into your phone and connect a mouse to it. You can connect any kinds of corded or wireless mice; to illustrate the process, I’ve connected a corded one:

After that, a pointer will appear on the screen and you can use the mouse as usual:

Connecting a flash drive to a phone
Connect a flash drive to OTG cable and it will immediately appear on your phone as an additional section. Then, you can use it as a common phone memory, using, for example, ES Explorer program (or any other).

Connecting other devices to a phone
Besides the above-discussed devices, you can connect many others to your smartphone. For example, a keyboard, a joystick or even a printer. If you face any difficulties with connecting the certain device – leave a comment to this article and I will surely help you.
Good luck!