Linux – What does the -p flag do in “mkdir -p”

A noauto entry in fstab is one which, for different reasons, you do not want to have mounted automatically, at boot and with the mount -a command. It is mounted by specifying the device or the mount point explicitly, like in

   sudo mount /dev/sdb1

or

   sudo mount /home/MyName/MyMountPoint

The cases in which you do not necessarily wish a device to be mounted at boot are numerous,for instance when we are talking about a network device, which may or may not be available at boot time (you could be on a laptop, and away from home). Or it might be an encrypted device, for which you have to provide a password, and you want to have to do that only when you truly need that. And so on.

In fact, fstab is used to provide rules by which devices are mounted, whether at boot time or not.

Am I missing something?

When you invoke apt install … the command is apt, not install. man install will show you the manual for install which is a different command. The right place to start is man apt.

Some commands in a form foo bar anything … may have manuals available under man foo-bar. E.g. man btrfs-subvolume runs well, but btfrs-subvolume (as a command) doesn't exist, the syntax is btrfs subvolume ….

This is not the case with apt though; in my Debian there is no man apt-install. Yet man apt says install (apt-get(8)) where it explains apt install, so now we know we should read

man 8 apt-get

And this is it, -y is explained there:

-y, --yes, --assume-yes
Automatic yes to prompts; assume "yes" as answer to all prompts and run non-interactively. If an undesirable situation, such as changing a held package, trying to install a unauthenticated package or removing an essential package occurs then apt-get will abort. […].