My Illustrator files opens as XD files and I can't change it

I have to uninstall both and reinstall them again. Adobe is something that screw up with our lives but we still need them.

Settings > Apps > Default Apps

Scroll to the bottom and choose the "Set defaults by app" link.

Then choose Adobe XD from the list and click the "Manage" button.

In the File type and protocol associations select the .ai extension and choose AI from the list.

If AI is not in the list (this was my experience) open the windows registry editor and search for the .ai file type and see that the Illustrator version number corresponds to what is installed on the computer.

Changing the Illustrator's version number on registry worked here, thanks to joosdm20559267 for the fix tip.

Throughout last years, Adobe has been screwing our lives up instead of helping. Next time I'll think twice about proceeding with a freaking update, because they're not releasing updates, they're releasing ISSUES.

Here's the link so everyone can go vote on it in the official bug report threads:
XD / PS defaults to open .ai files - AI files should open with Illustrator only – Adobe Illustrator Feedback

Looks like Adobe is aware of it and working on it. But like many of you, I thought my head was going to explode with how infuriated I was to have my workflow (jam-packed with deadlines) interrupted to troubleshoot this stupid issue. That said, my temporary fix was this:
1) Did the registry hack where I changed it to Illustrator23
2) I uninstalled XD, cause it still seemed to be picking it up (MIGHT not be necessary), and this disassociated the AI extension from using that program.
3) Right-clicked on AI file and changed the default to Illustrator 2019. FINALLY WORKED.

I haven't tried restarting my computer, so I don't know if it will keep, but I'm fine leaving my computer running until Adobe sorts this out.

Adobe personnel is not always on these forums. Therefore, I have submitted a bug fix request to Adobe. You may wish to also.

Feature Request/Bug Report Form
Alternatively, you can vote what I submitted. It is titled: Illustrator 23 Not Updating Version in Registry on Windows 10, Therefore Associates to XD

Same thing happening to all our machines company wide and each of us designers workstations at home. It doesn't seem to be isolated but 100% an issue for 100% of us. All the "fixes" in this thread have not worked for any of us. For now everything has to be right clicked open with Illustrator or opened directly from illustrator. If this doesn't get fixed I would actually condone some sort of legal action towords Adobe. It would sure be interesting to track over all users, how much time over the duration this problem persists is waisted because of this fault.

So in my case, my psd and ai files were opening in xd. So the first thing I did was to uninstall all three apps and install them back again. That didn't work. So after following this thread, I tried changing registry and that also didn't work so, in the end, I did this:

1. I left XD instaled

2. uninstall PS and AI

3. instal PS and AI back again

And it worked. So hopefully it will work for someone else. Good luck.

This solution doesn't work for me. Now it's trying to open my AI files with the photo viewer. Ugh. I hate Windows. Only using it cause that's what they give us at work...

Same issue here (windows 10) with Adobe Illustrator opening by default in XD, and I've just found that all Photoshop files are also opening in XD by default and changing the option to use the correct program by default doesn't work either! Come on Adobe, where's the fix?

You don't have to necesarilly uninstall Ai and XD, I just followed the last steps from this comment and the issue was fixed, ty!