I've mentioned this before, but it still annoys me somewhat! Not sure if anyone can help, as it's almost as bad as a needle in a haystack. But here is my issue.
Even since I "moved" to Catalina on both of my Macs, I've had this somewhat semi-occasional issue on my Mac Mini (late 2012 model until February 1st; late 2018 Mac Mini since). There are times where when I start up the Mini, my browser, Brave, is launched. Even less frequently, my EMail program, Thunderbird, launches. I do NOT have it "designated" anywhere for either of them to launch when I start up the Mini. But this does not happen at all on my mid 2017 Mac Book Air! For both machines, I do have it designated that a small application called Memory Clean 2 launches. But it is not obtrusive at all, and again, the "phantom" launching of either Brave or Thunderbird happens only on the Mac Mini, never on the MacBook Air.
Additionally, I ALWAYS quit all my open applications before I shut down either machine.
When I "moved" from Mojave to Catalina, OS 10.15.3, as I usually do whenever I move to a new Mac OS, after making a SuperDuper! backup of the last. clean prior OS system, I restart the respective Mac from that backup, use Disk Utility there to Erase and Format the internal SSD as APFS, launch the installer file for the new Mac OS, do a fresh, clean, "virgin" installation of the OS, then migrate/copy needed files, folders, applications, settings, etc. from that just completed backup. I then restart the respective Mac, and everything is good. Because I use Tech Tool Pro, I do have to use it to create its eDrive.
I also very recently used the OS 10.15.4 Combo Updater to upgrade to OS 10.15.4, but the issue still prevails.
Some might suggest using another default browser to see if the issue goes away. That would be a possible "solution", but again, sometimes it's Thunderbird that gets launched. And the other quirk, as I've mentioned, is that this phantom launching on the Mini happens infrequently.
Finally, I keep my Macs "lean, mean, and clean", and in fact, am only using about 35% of the space of the internal SSDs ("empty" sizes 256 gig in the Mac Mini, 252 gig in the MacBook Air).
Anyone have any thoughts, ideas, etc.?