As far as I know, there is no support for flashlight while we are chatting via video call in a smartphone. Why it is not present? Is there any hardware limitation? Can someone explain?
It is explained in this Quora article:
Go to Settings and choose Flashlight / Torch.
During a video call press the Home button to access the light.
If none of those work a 3rd party APP can control the light.
Examples for iPhone and Blackberry are here.
The Mi forum question "How can i turn on front flash during video call - Redmi Y1 - Mi" says option #3 works. Another thread "[Q & A] How to start Selfy Light During Video Call????" claims it's a known problem and that you can fix it by resetting your phone and not enabling Face Unlock.
There's no reason the light shouldn't work and you can stand near a white wall or reflective surface to get enough light at night to make a video call. If you edit your question to include a specific phone model we may find a specific set of instructions for your phone.
If nothing works you can get around it by purchasing a "Phone USB Light":