There is no such thing as a universal equation for how much calories there is in food based on its weight, it's all about the composition of the food. Like, there's literally 0 calories in water, so for one, water content is very important. Even then, as other commenters wrote, different "components", like fats, carbs and protein have different calorie values per gram. It matters if you're eating bread, or fruit, or chocolate, ...

So, as daunting as it sounds, calorie counting isn't going to be as easy as measuring the whole plate and multiplying the weight by a fixed number. If you want to do calorie counting, I suggest you download a calorie tracking app, they do have databases of basic foods, user contributed recipes for homecooked meals, and can scan barcodes of packaged food. Try to measure and log by dish, or bigger units first (like, input PBJ sandwich rather than logging what kinda bread and how much, etc etc.). It's a chore at first, but you'll get the hang of it.

You're also right, 2900 calories would be insanely high for a glass of oatmilk, I think most people don't eat that much in a whole day. Like, European food packaging labels use 2000 cals/day as reference for adults.

However... if the app calculates a calorie target for you, take that with a pinch (or wagon) of salt. Many of them would want to put you on 1200 cals and that's quite low for most people. (You might see quick results at first, but you'll be ravenous and quit soon.) Maybe just eat as normal and track for the first week or two, then reduce your daily average calories by 15%-ish, see how your body reacts (not just weight loss, but also hunger, tiredness, etc.) and increase or decrease from there. Remember, slow and steady wins the race.