Deep frying looks like a matter of hot oil and time, and it is a fairly precise thermal process where being outside the window produces predictable problems.
What is actually happening
Hot oil transfers heat to the food surface far faster than air does.
Water at the surface converts to steam and escapes, which is the vigorous bubbling.
That escaping steam prevents oil entering, which is why correctly fried food is not greasy.
The surface dehydrates and browns while steam protects the interior, which cooks by conducted heat.
Too cool
Steam production is insufficient to repel the oil.
Which means oil enters the food, producing the heavy greasy result.
Browning is also slow, so the food absorbs oil for longer before it is done.
This is the most common failure, and it usually results from adding too much food at once.
Too hot
The surface browns and burns before the interior cooks.
Which produces the dark outside and raw centre.
Oil also degrades faster at high temperature, breaking down into compounds that affect flavour and produce smoke.
The recovery problem
Adding food drops the oil temperature.
Which means the oil should start above the target so it lands in range, and batches should be small enough that recovery is quick.
Oil volume matters — more oil has more thermal mass and drops less, which is why a small pan with shallow oil performs worse than the quantity would suggest.
Measuring
A thermometer removes the guesswork entirely and costs very little.
The traditional tests — a cube of bread, a drop of batter — indicate a range rather than a temperature and are adequate with experience.
Watching for the oil to shimmer indicates it is hot and not how hot.
Choosing oil
Smoke point matters, since oil heated past it degrades and produces off flavours.
Refined oils generally have higher smoke points than unrefined ones.
Flavour matters too, since some oils contribute their own character.
And stability under repeated heating varies, with some oils degrading faster than others.
Reusing oil
Practical and requires attention.
Straining while warm removes particles that would burn and accelerate degradation.
Storing cool, dark and sealed slows oxidation.
Oil that has darkened substantially, foams, smokes at lower temperatures than it did, or smells rancid should be discarded.
The number of uses depends on what was fried and at what temperature rather than on any fixed figure.
Coatings
Batter and breading create a barrier that browns while the interior steams.
Which allows delicate items to be fried without overcooking.
Wet batters produce a smooth crisp shell. Dry breadings produce a rougher crunchier one.
Cold batter hitting hot oil produces more vigorous steam and a lighter result, which is why some recipes specify iced liquid.
Draining
On a rack rather than on paper, so steam escapes rather than condensing underneath.
Which keeps the underside crisp, and it is the difference between food that stays crisp for a few minutes and food that does not.
Salting immediately, while the surface is hot, helps it adhere.
Double frying
Frying once at a lower temperature to cook through, then again hot to crisp.
Which is the technique behind the best chips and much fried chicken, and the first stage can be done well in advance.
The mechanism is that the first fry cooks and dries the surface, and the second sets a crust quickly without overcooking the interior.
Shallow frying
Less oil, one surface at a time, and the same principles apply.
Which is more practical at home and produces uneven browning unless the food is turned and the oil is deep enough to contact the sides.
Disposal
Used oil should not go down a drain, where it contributes to blockages that are a substantial and expensive problem for sewerage systems.
Solidifying it and disposing with waste, or using a collection point where one exists, are the practical options.
Safety
Oil fires are the most common serious kitchen fire, and water makes them dramatically worse.
Which means never moving a burning pan and never adding water — covering to exclude air and turning off the heat is the response.
Never filling a pan more than a third and never leaving it unattended prevent most incidents.
Batter consistency
Thin batters produce delicate coatings and thick ones produce substantial ones.
Carbonated liquid produces a lighter result, since the bubbles expand in the oil.
Cornflour or rice flour in the mix produces a crisper coating than wheat flour alone, since they form less gluten.
Air fryers and oven versions
Neither fries, and both produce a browned dried surface through hot air.
Which resembles fried food more than baked food and does not reproduce it.
A thin coat of oil improves the result substantially, since some fat is needed for the surface to brown properly.
Which is why the results differ and why comparing them directly sets an unfair expectation.