What is this file and how can i access it?(.asset)

[bhaskarkumararya900]

It’s icon Is just as unity’s.

[AshwinTheGammer]

Its Scriptable Object. They are basically Data Containers or kinda Templates. You can create them from script. It’s pretty easy to create them.
Here is the link, if you want to learn more and want to create own:

Unity - Introduction to Scriptable Objects

Brackeys - Scriptable Objects