1 Open your Android's Settings. It's the grey gear icon likely found in the App Drawer.

2 Scroll down and tap Apps. This option is in the "Device" section.

On some Androids (such as the Samsung Galaxy), you must first tap Devicebefore you can tap Apps.
3 Swipe left on the "Apps" page. Doing so will open the "SD Card" page, which displays your currently used hard drive space in the bottom-left corner of the screen and your total storage space in the bottom-right corner of the screen.
4 Swipe left on the "SD Card" page. Doing so will open the "Running" tab which displays your currently-running programs.

5 Review the different categories. There are three values at the top of the page that will show you your Android's RAM usage:

System - The number of gigabytes currently used by your Android's operating system.
Apps - The number of gigabytes currently used by running apps.
Free - The number of free gigabytes of RAM.
http://www.ttechbay.com/2018/06/10/checking-hard-drive-and-ram-usage-on-android/

RAM is a valuable feature your device has, and it can determine how fast your device can be. However, if you want to learn how much RAM your phone has, you’ll need to enable the hidden Developer Options menu. This article explains how to see how much RAM your Android phone has.

https://www.novabach.com/how-to-see-how-much-ram-your-android-phone-has/