Where in finder can I see items on my startup disk (My Macintosh HD

Items under your userid are in your "HOME" folder.

You can add that folder to your Finder sidebar by checking the box with your name and a House icon in Finder preferences:

Customize the Finder sidebar on Mac - Apple Support (BY)

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MMinion wrote:

I want to maintain my internal drive as startup drive.

This is absolutely necessary for the M-series Macs or you will likely encounter problems at some point.

Its been suggested by the local genius, that with the identity on the startup drive, all it needs is the Application folder on startup drive and the remainder can be stored on external after re-pointing locations.

Do not relocate the home user folder at all or you will most likely encounter unexpected problems.

However, you can easily store any regular data files onto other media (external USB/Thunderbolt drives, network shares, etc.). You can also relocate the various macOS Libraries (Music, Photos, Videos, etc.) to external media since Apple provides instructions for doing so which means it is an option which Apple currently supports. See this Apple article for details...there is mention of Photos & Videos/Movies part way down the page:

Free up storage space on your Mac - Apple Support

Change where your music files are stored on Mac - Apple Support

the finder's GO menu has a lot of places you can choose quickly, including your Home folder:

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If you go into Finder > Settings… and click on the General tab, you can choose which items to show on the Desktop.

When Macs first came out, they always showed icons for disks on the Desktop. Some time during the Mac OS X / macOS years, Apple decided to get cute and hide icons by default. If you go into Finder Settings / General – then check off [X] Hard disks and [X] External disks – you can get the old behavior back.

(Macs don't ship with hard disks any more, but the Finder still uses the term "Hard disks" in its dialog, and calls the internal startup disk "Macintosh HD".)