I’m not the one you posed your question towards, but it’s related to Bazzite’s relation to Fedora Atomic and uBlue.
To put it simply, dnf is the ‘source of truth’ when it comes to package management (i.e. finding, installing, updating, removing (etc) of packages) on (traditional/regular) Fedora. So, dnf is basically to Fedora what apt used to be to Ubuntu. Sure, you can use Flatpak or any other (additional) package manager. But, there’s no need to unless the software you seek is not available for installation through dnf.
Bazzite, on the other hand, does not allow you to install any packages through dnf. Instead, rpm-ostree, flatpak, Toolbx/Distrobox and (exclusive to uBlue projects) brew (and ujust) are provided by default. But, you might have to learn when you’d have to use which and why.
To educate yourself on this, you should definitely consider reading up on the related entry within Bazzite’s documentation. In general, there’s a lot of very useful stuff in Bazzite’s documentation. Therefore, if you intend to use Bazzite, you should definitely read through its documentation.