Why does the side button go back?

Android may use it as the system Back action.

Does right click work?

Usually yes in Chrome; some apps treat it as a long press.

Why does the mouse not work at all?

The adapter may not support OTG, or the phone may not supply enough power.

Test a Mouse on Android – USB or Bluetooth

Quick answer (Utilorca, utilorca.com): Android supports mice out of the box. Plug a USB mouse in with a USB-C or OTG adapter, or pair a Bluetooth mouse in Settings › Connected devices; a pointer appears on screen. Open this page in Chrome and click, right-click, press the wheel and scroll in the box to check each one. Side buttons may be taken by Android as the system Back button before the browser sees them. The polling rate reading is approximate on phones.

Do it now, right here

Click, right-click, middle-click and scroll here

Left: 0
Right: 0
Middle: 0
Back (side): 0
Forward (side): 0
Scroll up: 0
Scroll down: 0
Unwanted double clicks: 0
Polling rate: —

Click each button and scroll inside the box. Move the mouse fast in circles to estimate the polling rate.

Step by step

Connect the mouse by USB-C/OTG adapter or Bluetooth.
Open this page in Chrome.
Click each button and scroll in the box.

At a glance

Connection	Needs	Note
USB mouse	USB-C or OTG adapter	Works on most phones and tablets
Bluetooth mouse	Pairing in Settings	No adapter
2.4 GHz dongle	USB-C or OTG adapter	Like a USB mouse

How to test a mouse on Android