would a light meter app on my phone work the same as an normal light meter?
Generally, this works fine (especially for negative film). I have a light meter app I use on my Pixel 7 pretty regularly; it lets me meter an average of whatever the phone's camera is seeing, a smallish spot (seems like about 5 degrees), or a "sensor" view from, I think, the selfie camera (not very sure, I haven't used that mode much). I've gotten good results with both B&W and color negative films using this app with a variety of cameras and films.
will my camera sensor size being four thirds change the out come if i copy the settings onto my kodak camera?
As long as the camera is set to the same ISO as the film you loaded into the Retinette, yes. You may or may not have manual control of ISO setting on the digital camera, however (the more auto-everything the camera is, the more likely this is to be an issue).
always thought that if the background was blurred it would be within portrait photography
Background blurring is due to something called "depth of field" -- I can shoot a portrait with foreground and background objects (up to a point) in focus, or set up so the focus has so little depth that when I focus on the near eye, the nose and cheekbone and ear are visibly blurred. For actual portraits, of people, I usually try to go somewhere between these extremes.
For bird photos, as others have mentioned, a Retinette isn't the best choice of camera, but if you have some birds that are used to coming to a window feeder and being watched from behind the glass you might get some good images. My first adjustable camera (also my first 35 mm) in 1972 was a Kodak Pony 135 with a closest focus distance of just over a meter -- but while I owned it I made some really nice macro images of tiny flowers, using a push-on closeup lens and some diopter arithmetic learned for the occasion. Almost no one who saw those images would believe they were made with that little, old (even in 1972) consumer camera, by a twelve year old kid. If you load ISO 400 film, you can still open up the lens enough to get some background blurring when near minimum focus, and you'll be able to use your highest shutter speed to mostly freeze the wing beats (at least of slower birds like pigeons, robins, and crows -- hummingbirds need a setup that can stop a bullet in flight).
For a question no one else seems to have addressed -- when you get an exposure setting, whether from Sunny 16, a light meter app on a phone, or an actual light meter (Reveni, among others, are making new light meters at somewhat reasonable prices, or it does work to transfer settings from a digital camera as long as you can control the ISO), you can shorten the shutter speed by a factor of 2 for each time you open the aperture by "one stop" -- which go by the square root of two (1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 22, 32, etc.). If you have f/16 at 1/100, you get the same total amount of light on your film with f/11 at 1/200 or f/8 at 1/400 (and the difference between 1/100 and 1/125 or 1/400 and 1/500 is trivial; it's 1/3 stop and even slide film will tolerate that much difference). This relationship between aperture and shutter speed starts to break down when the shutter goes longer than about a second, with most films, but for anything you're likely to want photographing people or animals you'll be fine.
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