Fedora shows / mounted at the same location as home

When I add a large file to /home, I see the used space increasing in both.

This is how btrfs works. You have one partition formatted to btrfs and the filesystem itself is divided into multiple (in case of Fedora two) subvolumes. All the subvolumes share the same space, that's why you see both / and /home having same 814G free space and that's why creating a new file in /home also increases used space in /. But there's no reason to be worried, it's still two separate directories and you can't overwrite data on / when writing to /home or vice versa.

While installing it I created separate partitions for / and /home

If you used the manual partition tool and selected btrfs (which is now default) you created subvolumes, not partitions. If you want separate partitions, you need to switch the partitioning scheme from Btrfs to Standard Partition:

This is normal when you mount different subvolumes of the same Btrfs filesystem. In the output of mount you will see subvol=… different for each. Your /etc/fstab is probably where these are specified.

I created separate partitions for / and /home.

Maybe you did create partitions, but somehow these mountpoints use a single filesystem now. Or maybe you misinterpreted and it was about separate subvolumes from the start? Without details I cannot tell. You may want to investigate if there's an unused partition.

Compare this answer of mine. I think it may be helpful.

Yes, mounting subvolumes like this is safe.