Desktop Linux isn't secure by default. Most apps aren't strongly sandboxed, plenty of packages have broad access to your files and session, and a lot of the actual security comes down to how you configure things. You can harden it with full-disk encryption, sandboxed apps, SELinux or AppArmor, and regular updates. But treat it as a system you can make more secure, not one that's automatically secure. What’s amazing about Linux is how much control you have over it.