How can I write "stuff" into USB drive on FreeBSD such that, the "stuff" is accessible in Windows [yes my wife still uses that :( ] and Ubuntu?
Windows can only mount FAT variants and NTFS natively. Non-native filesystems exist but require going through hoops to the point that it's often simpler to install a Linux (or FreeBSD) VM. So, pick FAT32 or NTFS.
Sure. Just format it with
newfs_msdos (you can even format whole drive, without any partitioning scheme).