Seriously! Can anyone convey how to update my mac to a MacOS 10.13 ??? Or is it a trade secret?

Depending on the hardware build model and year,

the computing device may, or may not actually be

able to 'upgrade' to a newer one at all.

How to download macOS - Apple Support

//support.apple.com/en-us/HT211683

So if you could identify exactly what is missing here

then perhaps someone could help. ~ I'm serious!🤪

..moo.jam.boo..

Depending on exact details of any product(s) you were to choose

any 'mix match of older' outdated kit ~ might be like chasing clever

rabbit down 'sizable' money hole; where your exit fees only grow.

Amazon? Maybe see what OWC has, & after you get some ideas

maybe after the holiday rush, call and talk to a product specialist

about their Macs. External displays, & how adapters/cables and/or

accessories, might not be good to get before you know a lot more.

An authorized reseller of replacement hardware, update/upgrades

and guaranteed too, may have some flaws; but are good & helpful.

(Doubtful if your vintage is beyond those of us who've replied so far!)

An all new M2 mac with extended applecare+ is my recipe suggestion.

+1! 👍