I think flat stomachs are genetically inherited, honestly, OP. I've never had a flat stomach, and I've always been slender. I can see 4 separate ab muscles - the top ones. Then right below there's pudge

Once in my late 20s, out of curiosity, I slimmed way down, exercised like crazy, and nope, still couldn't see the two bottom abs. But since I have a slim figure generally, I wear high-waisted clothes and no one is the wiser.
I've been told that there might be a link with auto-immune issues, and I do indeed have two auto-immune diseases.
The best you can do is eat clean, exercise and reach your target weight. After that, if you still have a belly, well, you'll have to learn to live with it.