Sorry if I made it sound complicated! Try googling "Linux file layout" - this link comes up first
Everything in Linux is a file (even a hard drive) if it isn't a file, its a process.
And I urge you to read "Linux in a Nutshell" which explains it all much better than I do!
/ is just the base of a file structure which contains files which may be devices. /boot will be a separate partition (usually). the /partition does contain the mountpoints for the filesystem, and usually contains the actually files for that mountpoint, but it doesn't have to. / could be a very small partition (say another 100Mb and all the other branches below that (/etc, /opt /etc /root. /home, /bin, /sbin, and so on could be on their own partitions or drives, although apart from /var and /home (and perhaps one or two others) there is no real reason to do so unless those file systems are very big - not something you would encounter on a home system.
Think of the computer boot process
Processor starts jumps to bios, bios sets up interrrupt vectors, and control jumps to the first sector of the first hard drive. That executes, and in linux starts grub (the boot loader) which continues executing the code in the boot partition. This will create a tempoary ram disk, and load a kernel image to it which will include driveers for your software raid and/or LVM. Then that will bring up the software raid, transfer control to the real kernel and mount the file system - which as I said, can consist of many partitions/physical drives.
As Aidenhjt says, how you partition is a matter of judgement. On my system my the partition that contains the majority of my root files, which is / the partition is around 10Gb. I elected to have /var on a separate (3Gb) partition because I back that up regularly, and /home on yet another partition because it is a shared filestore and I back that up separately. I use hardware raid, but I use Logical Volume management so I can resize my partitions dynamically.
/boot is about 100Mb and is outside the LVM structure for reasons I have described. The point is that /var /boot and /home are all mount points. If I unmount the /home partition, the branch is still on the file system (if I list / , I can still see /home - but there is nothing in it). I can creat a mount point /fredblogs and mount the partition there - I will then see the files in my home directory (/home/peter) at /fredblogs/peter. Of course some links would break so I wouldn't normally do that - but where I do use that is in backing up. LVM allows me to take a snapshot of a mounted filesystem, so I take a snapshot of /var - mount at at a mountpoint I created called /varback, and then back up that snapshot while /var is being accessed by the system. Nothing acesses /varback other than the backup software.
The reason for that? I have adatabase in the /var filesystem and backing up databases while they are running is not a good idea (the backup can be inconsistent) , but this way the backup filesystem (varback) is static and the database files are consistent.