When installing to free space created with diskmgmt.msc from Windows 10 for dual boot, can Ubuntu just install everything to that free space as one partition? Or, is it required to create /home and /root? From what I read, swap isn't really required unless there's low RAM.
You need at least one partition holding everything. Making separate partitions for
/home,/srv,/opt,/varor swap is optional and depends on your preferences and intended use of the system. Some swap space is just about always highly recommended, but Ubuntu is perfectly able to use a swap file just like Windows; a separate swap partition is not needed and future Ubuntu versions will no longer create it by default.After having made unallocated space, the Ubuntu installer would normally take all of that free space, unless you tell it otherwise during the install process.
The number of partitions used is moot, because your Windows 10 system probably uses GPT disk partitioning, and you can have up to 128 partitions. If however, your disk is formatted in MBR, then the max is 4 partitions, and Ubuntu installer will probably opt to install all of Ubuntu into a single extended partition.
Don't set up separate / and /home (and /whatever) unless you have a very good reason to do so... and especially if you've only set aside a small amount of disk space for Ubuntu.
Swap space is highly recommended. (Either swap partition or swapfile).