Merging two disks to one? I installed MacOS on disk 0. I have one HDD drive which is split into multiple disks. Now I would like to merge disk 0 and disk 1 (same hdd) to one disk. Is it possible and what should I do? For illustration I have added a screenshot with the current partitioning.
You can use Disk Utility.

Personally, I would copy all data from each partition to another storage device; creating a parent folder for each partition (using Finder); or, create a disk image (.dmg, etc.) file using Disk Utility.  Then, you can recover if anything goes wrong.  Here is an article describing the different types of disk images, how to make them, and why one might choose one over another:
https://support.apple.com/guide/disk-utility/create-a-disk-image-dskutl11888/mac

But, if you intend to merge two partitions, both containing data, relying on a software process which cannot be undone (sans great expense, difficulty, and uncertainty)... you are braver than I am.

Here is an article describing the different partitioning options:
https://www.lifewire.com/disk-utility-add-delete-resize-existing-volumes-2260078

If you want to run any Erase security options, you are better off running them on each partition BEFORE deleting each.  But, if the HDD is a SSD (or FLASH), I believe those options are irrelevant.  Wherever you see "APFS", that volume is on a SSD (using MacOS 10.13 or later).

If either partition is bootable, you will need to make it bootable again.  I noticed that "Install MacOS..." is one of your partitions.  Here is an article describing how to use Terminal (sudo... createinstallmedia command) to do that:
https://support.apple.com/en-us/HT201372

But, I have never attempted to use a bootable installer partition for "additional storage"; may not work, or have unexpected results.