A phospholipid molecule has a water-loving, hydrophilic head at one end and two water-fearing, hydrophobic tails at the other, both parts of the very same molecule. When enough phospholipids are dropped into water, they spontaneously arrange themselves into a two-layer membrane, heads facing outward toward the surrounding water on both sides, tails pointing inward away from it, entirely without any outside instruction directing the process, and that spontaneous structure is exactly what every cell membrane on Earth is built from.
The bilayer forms spontaneously because it's the thermodynamically favourable arrangement
A phospholipid's amphipathic structure, water-loving at one end, water-fearing at the other, means the molecule genuinely has no stable, energetically comfortable way to sit fully exposed to water, its tails would be forced into contact with water either way. Arranging into a two-layer sheet, tails buried away from water on the inside, heads exposed to water on both outer surfaces, is simply the lowest-energy configuration available to a large enough group of these molecules, which is why the bilayer assembles itself automatically rather than needing any cellular machinery to build it.
That bilayer structure directly explains what can and can't cross a membrane on its own
Small, uncharged, nonpolar molecules can slip directly through the hydrophobic tail region with relative ease, while larger molecules and charged ions can't cross the same hydrophobic barrier unassisted, which is exactly why cells need dedicated transport proteins to move those specific molecules across. The bilayer also isn't a rigid, fixed structure, individual phospholipids move laterally within their own layer fairly freely, giving membranes a genuinely fluid quality, and cholesterol and other lipid molecules mixed into the bilayer further adjust how fluid or rigid the resulting membrane actually is.
What we're still unsure about
That phospholipids spontaneously self-assemble into a bilayer, and that this bilayer explains selective permeability and membrane fluidity, are well established, extensively confirmed cell biology. What's more genuinely disputed is exactly how membrane composition and protein arrangement vary functionally between different small regions of the same membrane, often called lipid rafts, some cell biologists treat these as real, biologically important functional domains coordinating specific cellular processes, while others argue much of the existing evidence for stable, distinct rafts is actually an artifact of how membranes get disrupted and studied in the laboratory, and that disagreement remains genuinely unresolved rather than a settled part of the standard membrane picture.
This sits inside Lipids & Membrane Biology, one of seven topics in Biochemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.