It looks like THIS should give you a pretty good answer to your question, as for your last question. 

Specifically,

$ sudo add-apt-repository 'deb dell.archive.canonical.com/updates xenial-dell public'
$ sudo add-apt-repository 'deb dell.archive.canonical.com/updates xenial-dell-service public'
$ sudo add-apt-repository 'deb dell.archive.canonical.com/updates xenial-dell-oem public'

Of course, you'll want to replace xenial with bionic.

As for what the difference is, it appears there is none. It's just a different server hosting the same thing.

Thanks for you reply, 

So all of the other codenames are not needed? I was thinking that internally dell know what all these code-names mean :

	bionic-sutton-base-timbuktu/	2019-02-26 05:42	-
	bionic-dell-whitehaven-mlk/	2019-02-21 13:12	-
	bionic-dell-beaver-pearl/	2019-01-22 13:22	-
	bionic-dell-berlinetta-cfl-r/	2019-01-22 13:22	-
	bionic-dell-wasp-n5-v5/	2019-01-22 13:22	-
	bionic-dell-brookhollow/	2019-01-16 04:01	-

Unless you know for certain that you need them, I would think they're unnecessary.

But it probably wouldn't hurt to add them if you're unsure. If you add the repository but don't use any of the stuff within the repository, all it does is take up space in your cache.

Ubuntu 18.04 released in April 2018 and will be supported until April 2023.

I have no issues with current 18.04.2 vanilla install and my Dell G7 7588

https://wiki.ubuntu.com/BionicBeaver/ReleaseNotes/ChangeSummary/18.04.2

Ubuntu 12.04's LTS period ended April 28, 2017, you can purchase an Ubuntu 12.04 Extended Security Maintenance (ESM) license. Unlike the newer versions of Ubuntu, these updates won't be free.

http://releases.ubuntu.com/18.04.2/

64-bit PC (AMD64) desktop image

64-bit PC (AMD64) server install image

Ubuntu 12.04 patches will be available only through the Ubuntu Advantage support plan. Prices start at $150 per year per server and $250 per year with a minimum of 10 virtual servers. The latter plan includes work week online and phone technical support. There's also a desktop plan, which will run you $150 per year per desktop with a minimum order of 50 desktops. These updates will be delivered in a secure, private archive available only to customers on a per-node basis.

ESM subscriptions will last for at least twelve months.