You get only one boot partition (EFI partition) which contains the kernel and the initramfs for both operating systems. Then, you would create two partitions to hold the rest of each individual operating system.
Shared partitions can be ext4, but if they should be read-/writable by windows, I would recommend ntfs or exFAT.
If the disk is internal and only used by linux, you should 100% use ext4.
NTFS is what windows uses. exFAT is like really, really old file system that is only used because of its wide compatibility nowadays. USB-sticks use them, because they have to be compatible with any device where you couls potentially stick them in.