Resizing partitions
Why the fuck is there no one that can tell you how tf it works?

How to do it
Preperations
You need to be in a rescue system or have the disk unmounted. That is non negotiable!
Also make a snapshot/backup of you system!
Resizing
e2fsck -f /dev/sdXY
resize2fs /dev/sdXY XG
fdisk /dev/sdX
In fdisk you need to:
Delete the old partition (d)
Recreate it (n)
Use +XG to resize it to the GB amount you choose when using resize2fs
Write it (w)
Now you should have a working partition that is resized to the amount you wanted.

How to online grow a ext4 partition
This tutorial uses Arch Linux so package names might vary
Install cloud-guest-utils which includes growpart
Backup your partition table so you can recover if needed
sudo sfdisk -d /dev/sda > partition_bak.dmp
Use growpart to grow the partition
growpart /dev/sda 2
Use resize2fs to grow your filesystem
resize2fs /dev/sda2
Now you should have a ext4 partition that spans the entire rest of your drive.