... whenever I install or update something it just fills up my / firectory and i somehow cant use that 99 GB in my /home
Yes, because packages are installed systemwide. A package manager should never touch anything in /home.
Note that 20GB is sufficient for a majority of regular desktop users (which is likely why it might be a default in archinstall). You may want to look into what's taking up all that space. If you recently installed and that's filling up, that's most likely indicative of a problem you'll want to solve rather than just buy more time by giving it more space to fill.
There are some use cases that would / should need more than 20GB, but these would be due to specific huge packages (e.g., tensorflow and 0ad are some of the bigger beasts that come to mind). If you are not aware of any particularly huge optional software that you have installed, then there's like a problem to be found / fixed.
For one data point, while I definitely run a pretty slim system, all of my arch machines only use around 3GB of the root partition.
]]>Also, resizing with GParted also doesn't work, it's just kinda capped at 20 GB
Firstly reduce the size of /home Partition to free up some space, the reclaim that space into / (Root Partition).
]]>Use GParted Live
MAKE BACKUP FIRST!!
]]>Moving to 'Arch Linux Guided Installer'.
]]>Edit: I also already tried resizing it trough the "disks" app, but it's stuck at 20 GB
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