Ghidra navigation card. Not a screenshot. You do not need Ghidra for this page.

From Getting Started, effective as of Ghidra 12.2 and subject to change:
Linux is a supported platform.
The stated software requirement is a Java 25 64-bit JDK.
Install by extracting the distribution. Do not extract it on top of an existing copy.
GUI launch is ghidraRun from the install directory.
An official public release includes native binaries for Linux x86-64.
Linux ARM 64-bit is listed as needing user-built native components. Building them is not part of this lesson.

From the Decompiler window help:
The window shows one function at a time.
It follows the current address in the Code Browser.
Decompilation is done on the fly and picks up labels, data types, and comments the user has applied.

Finding main, without a project:
If a later owned project shows a function named main, that name is a symbol in that file.
A missing name does not prove where main is.
The program entry and the C function named main are different ideas.
The GDB manual says that for C the main procedure is named main.
This course has no Ghidra project, so no symbol was looked up.