How to open a Canon "RAW" file into Adobe Camera Raw editor from within the Lightroom cc`

I was not being a troll when I asked why, Just cautious as to not assume.

But I can not let well enough for now alone. My bad. I am going to make an assumption and perhaps miss the whole question. I will assume you do not like the RAW conversion via Lightroom Classic. One large group of users that fall into this camp area Fujifilm X camera owners. And the use alternative RAW converters. As you may see in my signature below, I have an X-T3, so I am aware of that camp.

Well, PROBLEM, Lightroom Classic is Adobe Camera RAW built around a Data Access Management program. And going around Lightroom and using Adobe Camera RAW converter will provide exactly the same results. You will just wind up with an additional photo, be it a PSD, TIFF, JPEG, or DNG with the exact same problems.

Now, as you may know, you cannot tell Lightroom Classic to go and use a different RAW converter, Adobe Camera RAW is hard coded into it, no option. No preference item exists to do so. I am sure any feature request fir that have been ignored.

What you can do is use a third party RAW converter outside of Lightroom, save the photo, then import the new photo into Lightroom. Typically these new photos are DNG. And yes, an extra copy of your photo. For example many  Fujifilm X camera owners absolutly hate Adobe Camera RAW and instead use Iridient-X-Transformer. (or dump Adobe all together and use something else) In Iridient-X-Transformer, you alter  as you see fit several settings, you define where the new DNG files will go, then you select what images, and walk away while the process occurs. Later you Import the DNG files (ADD) and move on.

Now you could use Adobe Camera RAW to do this, some try by just running it, some via Bridge. But what would be the point?  You could also also use the free Adobe DNG converter, but that is actually Adobe Camera RAW within a tool to open your photo and output a DNG, so, once again same RAW conversion as in Lightroom (thise not willing to change to subscription, but with new cameras not supported by LR 6.14 etc, might use this)

 One additional assumption, probably bad. A desire to accomplish work in Photoshop, then in Lightroom. That is backwards. And this assumption so bad, or the answer back so wrong, that I will line thru it (not delete, who knows) 

 But then, did I get why wrong?

Here is an interesting concept-

PHOTOSHOP SMART OBJECT LAYER LINKED TO LIGHTROOM EDITS

You can use Lightroom to edit DNG or proprietary RAW image files when placed as a "Smart Object layer" in a Photoshop image document (.PSD), but it involves a work-around method.

1. You need to save Lightroom develop edits to the raw file as .XMP  (Ctrl+S)

 (Proprietary raws need attached XMP, DNG raws accept metadata embedded)

 “Automatically write changes to XMP”  is an option in Preferences.

2. In Photoshop you need to "Place" the image into an existing open document by- Menu: {File>Place Linked}.

You need a background layer eg. Create a new blank PSD, or open any other image (eg.  a blank document from Lightroom).

(Older Photoshop versions require the 'background' image to be the exact same pixel dimensions as the 'Placed' image. New PS versions 'Place' the image layer with 'Transform' active.)

For File>Place Linked It is necessary to navigate folders to 'find' the name and location of the image to be 'placed'.

3. When 'Placed' -ACR will open with the Lightroom edits applied. Click [ Open ] to accept the image in ACR.

You can then at any time double-click the 'Placed' layer thumbnail to Edit in ACR, which will write its changes to the .XMP metadata.

4. Back in Lightroom you can then update the LR view by reading the XMP metadata-
Menu:  {Photo>Read Metadata from file}

5. Further develop editing in Lightroom requires a "Save metadata" (Ctrl+S) once again to update the linked layer in the PSD file when it is edited.

Lightroom-Classic IS Camera Raw!  Just a different UI. (Lr with extra benefits!)

There would be no purpose in LR>ACR>LR.

Why?

ACR (Camera Raw) is an integral part of Lightroom, so it makes no sense (neither is it possible) to open an image in ACR from Lightroom.

But you can send a raw file to Photoshop for editing (Right click > Edit in, or Ctrl/Cmd + E).

The file will then silently open in ACR, before it opens in Photoshop. When you save the file in Photoshop, it will be saved as a Tiff or PSD, according to what you have chosen in Lightroom preferences > External editing.