AWS EC2 EBS: btrfs partition and FS automatically resized by just increasing the EBS volume size?

I had to expand the size of an EC2 EBS root volume that uses btrfs (Fedora 38 Server), so I did the following:

1. Stopped the EC2 instance
2. Expanded the EBS volume via the EC2 console
3. Started the EC2 instance again

Then I was ready to play around with partition resizing and filesystem resizing inside the EC2 instance, but, to my surprise, the system now claims to have done everything automatically. The btrfs partition (nvme0n1p5) seems to be automatically expanded, and the filesystem seems to have the expanded size as well. If it works I'm not complaining, but what happened here? How did the partition get resized and how come btrfs claims to be able to use all the newly added space, all automatically?

lsblk (post-expansion)

NAME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
zram0       252:0    0  3.8G  0 disk [SWAP]
nvme0n1     259:0    0   50G  0 disk 
├─nvme0n1p1 259:1    0    1M  0 part 
├─nvme0n1p2 259:2    0 1000M  0 part /boot -- ext4
├─nvme0n1p3 259:3    0  100M  0 part /boot/efi ---vfat
├─nvme0n1p4 259:4    0    4M  0 part 
└─nvme0n1p5 259:5    0 48.9G  0 part /home  -- btrfs subvol
                                     /      -- btrfs subvol

btrfs (post-expansion)

$ sudo btrfs filesystem usage /
Overall:
    Device size:                  48.92GiB
    Device allocated:              5.74GiB
    Device unallocated:           43.18GiB
    Device missing:                  0.00B
    Device slack:                  3.50KiB
    Used:                          2.63GiB
    Free (estimated):             45.89GiB      (min: 24.30GiB)
    Free (statfs, df):            45.88GiB
    Data ratio:                       1.00
    Metadata ratio:                   2.00
    Global reserve:               11.00MiB      (used: 0.00B)
    Multiple profiles:                  no

Data,single: Size:5.23GiB, Used:2.52GiB (48.17%)
   /dev/nvme0n1p5          5.23GiB

Metadata,DUP: Size:256.00MiB, Used:55.95MiB (21.86%)
   /dev/nvme0n1p5        512.00MiB

System,DUP: Size:8.00MiB, Used:16.00KiB (0.20%)
   /dev/nvme0n1p5         16.00MiB

Unallocated:
   /dev/nvme0n1p5         43.18GiB

I'm not sure of your exact configuration but "cloudinit" is able to resize a BTRFS filesystem if the partition size is extended.... probably it ran when you booted the machine after resizing.