When installing Ubuntu, it will decide what kind of partition to use:
Ubuntu decicison tree for using entire disk:

? installer booted in EFI mode or BIOS mode?
  * EFI
    : use gpt and never make biosboot
  * BIOS
    ? is the disk larger than the max for msdos (2TB)?
      * yes
        : use gpt and ensure there's a biosboot partition
      * no
        : use msdos

ref

When using free space:
Ubuntu installer will use the existing partation.
Ubuntu installer will automatically create a BISO Boot Partation (1MB, code EF02) for GPT if installer is booted in BIOS mode.

If win10 is installed in UEFI mode, it can be booted when mode is changed to BIOS (sunnycude tested, but still weird, maybe due to Intel motherboard). However, new Ubuntu installer in BIOS mode will not recognize it. Finally, Ubuntu itself cannot boot due to compatibility problems with Intel SSD.