virtualbox – How to virtually install use test custom or stock ROM safely?

My mobile device have

OS Android 6.0.1 Marshellow Processor Qualcomm Family cortex a53
Machine arrch 64 Architecture arm cortex a53 External memory card
64GB ROM 16 GB MOBILE NOT ROOTED

While finding twrp stock from for my mobile device I founded following custom ROM for my mobile

  1. Android 9.0 Pie:
  2. Lineage OS:
  3. AOSPExtended ROM:
  4. Resurrection Remix:
  5. Carbon ROM:
  6. MIUI:
  7. Pixel Experience ROM:
  8. DotOS:

Now as a novice and new to a droid Linux programming I just knew the difference or custom and stock to.is that the stock from is made my manufacturer and custom ROM is made modified created version by private android programmers developers.

When Google founded about

DUAL MULTI BOOTING Apps

Googled and founded many apps tools and articles but confused don’t know which one is safe and secure tool way to dual boot and how to use those tools apps

But some limited devices are supported with these tools will they support in my mobile device or not

  1. BOOTMANAGER
  2. XDA DUAL BOOT PATCHER
  3. MULTIROM & VIRTUES

https://www.xda-developers.com/dual-boot-on-android-a-power-users-holy-grail/amp/

  1. efidroidmulti

  2. MULTIROM

How To Dual Boot Multiple ROMs on Android Phone

  1. TWRP multiboot

VIRTUAL ROM TOOLS Apps

Googled and founded many articles apps on virtual ROM but confused don’t know which one is safe and secure tool way to dual boot and how to use those tools apps

But some limited devices are supported with these tools will they support in my mobile device or not

  1. VMOS

2.VirtualBox

  1. ROM Manager

  2. DRIVE Droid

#Now My queries#

  1. Is custom ROMs safe and worth does chances to harm flashing customs rom

  2. HOW TO PRE TEST THE BOOT CUstom ROM BEFORE INSTALLING virtially weather they are working properly or not and does it have good featires

  3. As i know in mobile android is booted from ROM or internal memory. Have seen video article to SWAP external memory card as internal memory. Means can we swap cards and install OS in external memory card. Or from UB virtually

  4. Dont know if its possible to multi boot it bot form.is not external as card or not.Want to do same in my smart phone with dual booting of OS of same or other versions in memory card or internal memory

  5. So in android I want a boot menu in my smart phone through which I/can boot to android 5.1.1 or android 6 or 7

  6. If Dual booting not possible then can I VIRTUALLY install or boot different boot ROM through any app kindly tell me which app is good and secure for virtual loading stock ROM

  7. Will dual booting or virtual booting will be harmful or may can damage Mobile device or harmful is there any safe secure method through which I can do it

Kindlly share me links of step to step through which I can learn steps for dual booting or virtual booting as I am novice in android installation

For safety I will created images backup of original stock ROM through adb fastboot commands. TAken backup of Boot system userdata and mmcblk0 to recover if any problem.exists except if don’t harm.hardwares.

Waiting for a professional experts advice and thanks a lot for your precious time and advice’s and help