using 2019 imac as monitor

Sorry, no can do...

The following older iMac models can be used as an external display when plugged into another older Mac model.

iMac models introduced in 2011, 2012, 2013, and mid 2014

To use any of these iMac models as an external display:

• The iMac used as a display must have macOS High Sierra or earlier installed.
• The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
• The cable connecting the two Mac computers must be a Thunderbolt  or Thunderbolt 2  cable.

24-inch and 27-inch iMac models introduced in 2009 and 2010

To use any of these iMac models as an external display:

• The iMac used as a display must have macOS High Sierra or earlier installed.
• The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
• The cable connecting the two Mac computers must be a Mini DisplayPort  cable.

Use your iMac as a display with target display mode - Apple Support

27” 5K monitors might be ideal, but the Apple, LG, and Samsung ones are pretty expensive. One of those, plus an Apple Magic Keyboard and Magic Mouse might run you as much as a 27” 5K Retina iMac that included the computer!

However, if you get a 27” 4K monitor, you’ll find they are a lot cheaper. You may find that running one of those in Retina “like 2560x1440” mode works well enough, even if the Mac is downscaling an internal 5K canvas onto the lower-resolution 4K screen.

The 2019 iMacs don't support Target Display Mode. That feature disappeared when the first 27" 5K Retina iMac came out in Late 2014. Apple's Support document also indicates that the Mac Studio can't use any iMac – even one with the TDM feature – as a TDM display.

You could try setting up the iMac as an AirPlay receiver – and using it as a second screen, using AirPlay to Mac. Provided that you run Monterey or later on both Macs, they would seem to qualify.

System requirements for Continuity features on Apple devices - Apple Support

Set up your Mac to be an AirPlay Receiver - Apple Support

Note that AirPlay is a "software-type" connection. I don't know if it would use the full resolution of the iMac, or if there might some lags/artifacts you would not see with a real hardware monitor.

<<. even if the Mac is downscaling an internal 5K canvas onto the lower-resolution 4K screen. >>

As I understand it, the Mac is only downscaling the TEXT (to make it large enough to read) while the graphics are shown at the 'full native resolution of the display'.

Downscaling has the effect of making things smaller - not larger.

When my system is running one notch down from Larger Text, System Information shows “UI looks like” as 2560x1440 and “Resolution” as 5120x2880. The monitor shows that it is receiving a 3840x2160 pixel signal.

So are Retina-aware apps drawing photos directly onto a 3840x2160 canvas which is then merged with a 5120x2880 text canvas? (Which is what you seem to be saying.). Or do they draw a 5120x2880 level of detail and then have that diwnsampled to 3840x2160?

I wish there were one single document that laid out in plain language exactly what happens, and all would be well understood. That lack of an adequate explanation is why my response above has so many weasel-word in it.

"As I understand it, the Mac is only downscaling the TEXT (to make it large enough to read) while the graphics are shown at the 'full native resolution of the display'."

I think that means if you intend to run full motion Video on that display, even with a smaller TEXT "looks like" resolution, you get your graphics at full size.

My understanding is that a Retina-aware application uses the “UI looks like” resolution as a sizing guideline, but can then fill in photo areas with hi-res bitmaps having 2x as many pixels in each direction as the “UI looks like” resolution. The drawing doesn’t take place at native LCD panel resolution unless that just happens to match the pixel-doubled canvas resolution.

So there can be an extra level of resampling … though taking a drawing made at 5K resolution and downsampling it to 4K will produce better results than taking a drawing made at 2.5K (with 1/4 the number of source pixels) and upscaling it to 4K. It’s not like the really bad old days of sending 640x480 signal to a LCD monitor with a native resolution of 800x600 pixels and getting horrible jaggies.

On iOS/iPadOS devices, I think that the hi-res drawing sometimes takes place at 2x and sometimes takes place at 3x, depending on what is necessary to keep text readable for the device.

in any case, the goal is to use the high PPI of the display for drawing stuff in greater detail, rather than for merely cramming more and more smaller stuff onto the screen to the point of unreadability. That is often lost on people, especially ones who are coming from Windows and looking for excuses to bash Macs.

On Windows, there is a scaling factor that applications are supposed to honor, but my impression is that for a long time, many ignored it. Apple’s Retina approach provided a degree of backwards compatibility by letting the OS transparently map drawing calls from NON-Retina-aware applications into calls that would at least draw things at the proper size and position on the screen.

I hear what you are saying.

But if graphics are re-sampled for display, I would expect every-other posting on these forums to be a complaint about re-sampling artifacts, and there are essentially NO complaints like that.

I do not work in pre-press or video, so I am far from an expert on such matters.

+ 1 No.

You will need an aftermarket external display or an Apple display to go with the Mac Studio.

see > Connect a display to Mac Studio - Apple Support