Hello! I'm quite new to Linux. Sometime ago I installed Linux Mint and set up dualboot with win11 (both systems were on separate drives).

Today I tried to install Solus on the same partition of the same drive Mint was on, and it game an error. Then I erased all data on the drive and this way Solus installed, but...

But now when starting or rebooting, I have three boot options: Solus, EFI and UEFI. So, if I want to use win11, I have to choose UEFI and select win11 there.

The question is: how can I set things up so that I have both Solus and win11 as boot options on startup?

It sounds like you set up Solus as a legacy boot (bios) install. If I am right then you will most likely see a Grub screen displayed for a few seconds in the early stages of starting Solus boot.
Setting up your install media to support uefi and then reinstall Solus should fix this.
What install media did you use?
Some like Rufus need to be set up when building a bootable USB for either Bios (MBR) or UEFI (GPT)
others like Ventoy mostly seem to have the ability to be able to start up in either mode. If you have used something like Ventoy or a DVDR then you need to choose the UEFI option when booting from it as Solus will be available to install as a legacy (EFI) or UEFI (choose this option)
That is about the right information you need. There are plenty of very knowledgeable and helpful people on this forum who will no-doubt correct me if I have misdiagnosed your problem.
Good luck

    Okay, this is all too strange... Win 11 is installed to Samsung SSD, Solus is installed to ADATA.

    And yet I have Solus Bootloader on Samsung SSD? And whatever "ubuntu" is?

    I haven't tried booting either, by the way, I only used Windows Boot Manager and Solus Linux Bootloader on ADATA.

      Ezten
      I've seen this before, UEFI entries that don't go away. This is what I did:
      efibootmgr
      It'll show you those entries by numbers 0000 0001 etc

      efibootmgr -b #### -B
      To remove an entry. That might help narrow it down to just the ones you need.

        Ezten I used Mint's built-in Image Writer.
        I have never used that method I'm afraid so can't suggest anything. Sorry.
        As both OSs can be selected and run, is the slightly ugly selection route realy a problem worth spending hours on? Various Windows functions do tend to mess with Linux's continued ability to boot so there is a chance that there will be necessary maintenance in the future when you have more experience with the system.