Is there any relevant difference between Linux distros?

There are a few main differences between distros and distro families.

Package managers. Debian-based distros like Ubuntu, Mint, and Pop all use apt as a package manager. Red Hat/Fedora-based distros use yum. OpenSuse-based distros use zypper. In decades past, this mattered a lot more. Nowadays, they're all fairly robust and they all handle dependencies effectively.
Package repositories. Every major distro maintains its own package repositories, and some have different priorities. Some distros favor stability over cutting-edge features, only adopting software updates for security reasons rather than cutting-edge features.
Philosophy. Some distros take a hard line on only including free and open-source software in their repositories. Some take a more pragmatic approach, allowing some proprietary software like audio codecs or GPU drivers if you choose. Some favor minimalism for low-end hardware, and some throw in all the bells and whistles.
Update schedules. Some distros are "rolling releases", meaning they receive updates constantly. Most distros have a scheduled upgrade cycle, and some have "long term support" (LTS) releases. Generally speaking, rolling releases are more cutting-edge but potentially less stable, and LTS releases are the reverse.
Default configs. Even though basically all distros can run all the same software, they can preconfigure them differently. Ubuntu's Gnome may look very different from Fedora's Gnome, for example, even though they are the same software and could be configured the same way if you so desired. Also, some distros will default to BTRFS for the filesystem and others will default to ext4, and while you can generally use whatever you want on any distro, you may find that diverging from the default will make your life a little more difficult.
Third-party support. If you are in an enterprise environment, you're probably either a Red Hat shop or an Ubuntu shop, because those are the two distros third-party vendors typically support. "Support" in this case doesn't necessarily mean it won't run on other distros, but if something goes wrong, you're on your own. This can be a problem even in the consumer space. For example, I could not easily get UE5 to run in OpenSuse, and since they only officially support Ubuntu I was on my own.
Community support. This is particularly important if you're new to Linux. If you stick with a popular distro, you are more likely to get relevant results when you google something like "how do I install X on Y". There's a lot of information available for Ubuntu. Not so much if you're running, say, Justin Bieber Linux.

Similarly, there are over 400,000 species of beetle in the world. They're all different, but unless you're a entomologist, you'd be hard-pressed to tell most of them apart.

Tiny correction: Fedora uses DNF now, not yum (possibly RHEL too, but I have no experience there)
- Excellent point! This is especially important for laptops. If you are planning to install Linux on a laptop, I highly recommend searching for your specific model to find a distro with the correct drivers. Trackpads, wi-fi, and power management can be really finicky.
That's very helpful info to someone looking to step foot into the Linux world, like myself. Thanks!
Also Justin Bieber Linux LOL
Thank you! That's very through. I've tried a few distros on VM and as a noob with very easy to fulfill software needs I couldn't see the key differences between the ones I tried out.

It's not really a matter of perfection, more the fact that Linux isn't windows. It can be made to more or less look like windows, through the years, windows has borrowed lots of things from the Unix world, so windows users might think it's kind of familiar, but it's not.
Linux is its own thing. It's absolutely not a drop in replacement for windows. It can do a lot of the same things, but it won't do them the same way. And there are things windows will do that Linux won't and that Linux will that Windows won't.

Basically…. It’s flavor. A lot of it is a question of which package repository you’re using.

some of that is how quickly new features come down the pipes- some will push it immediately… some will stare at it hissing for a decade before deciding maybe it’s not so bad. (New features vs stability,)

Honestly, you’re choice in desktop enviro is more important. I grew up on KDE- mostly because my dad was a gnome guy and teens are rebels like that. There’s also cinnamon regolith and awesome, and you’re going to find those have a huge impact on performance as well as look and feel. (And can absolutely be swapped at will, even if kde plasma is… freaking huge…)

There is no such thing as "better" really. It's more about how much you want to tinker.

I ran arch and slackware 10-15 years ago. Now I have a job where I need to get actual work done so I don't have the time or energy for that anymore. I run mint.

Use something that fits your goals.

At a base level though, really there is very little difference. Any app can be run on any distro. Again, depending on how much you want to wrestle with things.

The difference between them is becoming less relevant but most of what you want from a distro are good defaults and stability. Some people don't want to have to make 100 changes every time they download a distro to tune it to one specific task, so you have distros like Nobara which is tuned for gaming and productivity, distros like Endeavor which are Arch-based but with a ton of things set up for you, distros like ZorinOS which are tuned specifically to be beginner-friendly and have helpful popups, etc.

I think it could be argued that most distros out there now are "just Debian/Arch but with [thing]" but I still think the distro choice is important to people who don't like messing with their system and want things to just work.

I still think the distro choice is important to people who don't like messing with their system and want things to just work.
I'd argue the reverse is true as well:
Distro choice is important for people who enjoy messing with their system and want to meticulously set up every piece.
- I think if people want to set up every piece they're better off downloading Arch and just installing packages they want

I mean looks isn't even really the thing, The main 2 things are default programs, and the package managers. IE arch based are good if you want the bare minimum, and for most packages to be the bleeding edge. Buntu based if you want the default packages to be more stable versions (at the drawback of not always getting the latest without setting up a repository).

Basically it's the installers and configuration tools that are the main differences. You are right that on a practical level if you ask me to make an arch system look like a debian or ubuntu system that's set up the way you like it, I could almost certainly make it barely distinguishable.

Default configuration
Package management
Release schedule