The data is stored in a separate partition
You won't find the Ubuntu system data in Windows. They are in a separate partition completely hidden from Windows 10.
A drive, a hard disk drive (HDD) or a solid state drive (SSD) is a physical object. What Windows people call a drive, such as
C:\ drive or
D:\ drive are partitions.in either inside a single drive or in two separate drives.
Ubuntu (any Linux) uses a different partition naming scheme
/dev/sda2 for two partitions on the same drive or
/dev/sdb1 for two partitions in two different drives. A default Ubuntu installation may use one or two partitions.
Windows 10 does not know anything about Ubuntu partitions
These partitions are formatted differently as compared with Windows partitions in a format called
ext4. Windows cannot see partitions formatted this way. In a default Ubuntu installation a partition (it may be
/dev/sdb1 etc.) is mounted at the mount point
/. To the user, the system partition is
/. This is something similar to
C:\ in Windows.
If you have C D E F partitions...
If you have a 1TB drive and 4 partitions C, D, E, F in Windows 10 each partition 250GB each, then you don't have any space to install Ubuntu. You will have to delete at least one of these partitions (not C, as you want Ubuntu along side Windows) or shrink one or more of them to make some unallocated space in your disk drive. Ubuntu installation will find that empty unallocated space and create the partition(s) it needs there.
After you install Ubuntu, formatting any of the Windows partitions C, D, etc. will not delete any data or system files in the Ubuntu partition
See Ubuntu Installation Root partition for more on the
See What is the meaning of the default directories in the Linux filesystem hierachy? for how the system files and personal data folders are structured within Ubuntu.
See What are the advantages and disadvantages of mounting various directories on separate partitions? for why one may mount various system folders as separate partitions.
Hope this helps