Technical differences:

Fedora uses RPM for package format, and is made to work with the latest versions of software, so it's almost a rolling release, and receives VERY constant updates (but it's still solid). The only other release model is the SilverBlue/Kinoite which is all about having an immutable base system and managing your applications through Flatpak.

Debian OTOH uses the DEB package format, and comes in 3 update models:

• unstable (bleeding edge software, breaks may occur) with constant updates
• testing, or Sid (with actively tested software, more akin to Fedora's main model. Stuff rarely goes wrong)
• stable (receives mostly security updates, focus on using battle-tested software versions. Ideal for servers and people who want their system to absolutely not go wrong. It's my current pick)

Project differences:

Fedora is on paper "community driven" but it's actually backed and steered on by RedHat. There's also a current proposal about implementing telemetry (turned on by default).

Debian is entirely community-made and driven, with no big corporation being its owner and/or main sponsor, and it has a stronger focus on FOSS. It's about as old as RedHat (both have their origins in the early 90s), so you can bet they'll both be around basically forever.

Edit: both are great distros, mature, stable and easy to use. Fedora was previously my most beloved, but my relationship with it soured over RedHat's leadership decisions. Don't let my current salt take away from the review :')