The main reason that you don’t need an “antivirus software” on GNU/Linux is that software is treated in a fundamentally different way. On windows, it is pretty common for people to download exe files from random website and run them. On GNU/Linux, you should not being running random executables that you found on the internet. The majority of the software that you use should be installed from your distro repository.

If you are very concerned about security, you can use a security auditing tool like Lynis. Lynis is a tool that I have used before. You run it and it makes a checklist of things that you can do to improve the security of your GNU/Linux system. It will probably tell you to set up an Intrusion Detection System like TripWire. It might recommend you to do something like having your system files on a separate partition and booting your system partition as read only when you use your computer normally. Most people don’t do all these things, but Lynis will tell you what is possible and you can decide what meets your security needs. Lynis is probably in your distro repository.

https://en.wikipedia.org/wiki/Lynis

This is a pretty good checklist of security practices for a GNU/Linux desktop system.

https://github.com/lfit/itpol/blob/master/linux-workstation-security.md