If you’re familiar with this process on a T8-S, it is exactly the same procedurally.

For everyone else, please watch a video

4 minutes 54 second is the relevant part (the rest is the How To Video for the T8-S Plus)
We’re interested in pausing at around 8 minutes 26.
Format internal HDD is the menu entry you want.

PLEASE MAKE SURE YOU REMOVE ALL USB STORAGE DEVICES BEFORE CLICKING THIS OPTION.
openELEC/Linux does not differentiate between internally connected and USB connected drives.

Click this menu entry and confirm your choice.
Continue to watch video after that to see how to switch back to Android if you prefer.
After the process has finished the drive will show in openELEC and Android (I’d recommend ES File Explorer, expand the LOCAL menu on the left). In the openELEC OS, it will be in the \VAR\MEDIA area (starting from root).