The main file is the fundamental code of your bot, making your bot go online and perform other actions automatically. Such as: index.js, bot.js and index.py, depending on what name you put in.
Exceptions (defaults)
Some bot making software, such as Discord Bot Maker and Discord Bot Controls, have a default main file already set up, which is: bot.js.
How do I know my main file?
JavaScript
The main file is the one you use to start your bot:
when running node MainFile.js command
which is usually in package.json in main
Python
The main file is the one you use to start your bot:
when running python MainFile.py command
by right clicking and then RUN in PyCharm.
Java
The main file is the one you use to start your bot:
when running java -jar MainFile.java command
by double-clicking the file MainFile.jar
Ruby
The main file is the one you use to start your bot:
when running ruby MainFile.rb command
Go
The main file is the one you use to start your bot:
when running go run MainFile.go command
Php
The main file is the one you use to start your bot:
when running php MainFile.php comma
Rust
Usually the main file is src/main.rs
The name MainFile.* is the file in the situation described, which varies from person to person since they can put whatever name they want, so analyze your method of turning the bot on by comparing it to the situations shown to know which is the main file.