Hi. It's been awhile. Lots of changes to linux.

I guess grub is vastly different then the last time I used it.
I'm using the lastest developer verision of neon

my grub file is confusing

GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=35 //changed value from 0
GRUB_DISTRIBUTOR='lsb_release -i -s 2 /dev/null || echo Debian'
GRUB_CMDLINE_LINUX_DEFAULT='"quiet spalsh"
GRUB_CMDLINE_LINUX="kept"

So I want to see the grub menu so can I change the timeout_style to show or true?
Then I tried looking up on the web and reading man files, looked at breeze and no configuration their.

I went to /dev/disk/by-path and see multi files. I'm just lost on what to do.
I tried grub-install --boot-directory=/mnt/boot /dev/sda and it doesn't work.

Can someone please show me how to add another os to the system. I tried hacking the system now for at least 2 hours. Thanks.

This is not really a Neon or a Plasma problem. You normally don't add OS yourself to Grub, this is done on OS installation. Every new Kernel version is added to grub when you update a kernel.

There are a lot of explanations out there on how to configure Grub, and not really a subject for this forum TBH. Maybe you need to change your search question to find the answer, but just googling your main question yielded a lot of correct answers in the first search page alone