There are some points regarding the attached image.
What happened?
The most important thing is, that your image is already underexposed by about 1.5 - 2 f-stops. This also means that your image is basically not ISO 1000, but rather somewhere in the 4000 region.
Why does auto exposure fail here?
The reason for the camera lightmeter underexposing your shot is that it is not that intelligent. It just looks at the image and tries to expose for what is sees. It sees a very light background and takes that into consideration, darkening the image. An intelligent choice would have been to know that the couple is the main subject and expose for them.
Auto exposure is working on even lighting and with standard subjects only. On any non standard lighting situation, it will try to compromise.
On any non standard subject, the same happens: photograph a black cat in the snow. Subject will be way too dark.
A white cat on a black sofa. Subject will be way too light.
How to correct auto exposure
You can counter this a bit by using mid emphasis light metering, so that not the whole frame is accounted for, but the mid is given more weight.
On mirrorless cameras like this, you can see the exposure exactly as it comes out. So it is up to you to correct the automatic mode.
For this you have a correction dial to counter any adverse effect that the situation creates. If you had noticed that the image looks too dark and set the dial to +1.5 the image would have looked ok-ish on the back display or the electronic viewfinder.
You can also switch on the histogram in the viewfinder to help you see if the images is too light or too dark.
What else can you do?
The other method of countering such situations is to add light to the scene by using constant lights or a flash. Note that just adding a flash without knowing how to balance ambient light with flash can worsen the image. You then get the "deer in headlights" effect.
In this location a flash pointing forward at the ceiling (opposed to directly at the subjects) might have done the job by adding ambient light.
But the image noise...
There is always more noise in the darker areas. If you expose the image incorrectly, you are inviting even more noise than you would expect. The only ways to reduce noise before the shot, is to add light by exposing correctly, adding artificial light or using a faster lens/smaller f stop number. The last option is sometimes not viable when photographing groups.
After the shot you can denoise the image via post processing - although there are limits before the image falls apart. Specialized denoising applications usually do a better job than generic image editors.
But the sharpness...
There is a number of things that can go wrong.
- Somewhat long exposure while subjects are moving,
- camera shake (although the R5 has a pretty decent stabilizer), If that was a telephoto lens, the 1/200 sec might be too long.
- Misfocus due to lighting conditions. The less light, the less contrast. And the contrast is needed for autofocus.
- If there are many people, eye/face focus can pick the wrong person.
- Cheap lens or converter with adapted glass that does not work that well (although the RF lenses are all pretty decent)
- Not the problem here -> aperture too small. Starting at f16 you will get softer images due to light bending at the aperture. Some lenses exhibit that earlier, some later.
- no contrast before converting to jpg / or no contrast and shooting in jpg. JPG compressions "summs up" pixels that have no contrast. So with no contrast to begin with, the compression is actually reducing sharpness even more by interpreting mushy areas as one big area and strips any detail left.
- using an APS-C lens on a full frame body. You only get the 45mp of the camera with full frame lenses. You can attach APS-C glass, which will net you around 17 MP. If you expect the same detail, this can be a pitfall.
Can you fix it?
Also the image might be salvageable - unless printed really big. See this edit which only had the jpg, so it is very limited. Some sharpening, +1.5 ev, some denoising. Done in Photoshop/Camera Raw in 30 seconds. If you shot this using raw files, there is more wiggling space for editing.
What else is there?
Also see these answers - there are a ton more regarding the two points sharpness and noise
