How breeding behaves in this game. Read straight from the build's settings, so it matches what actually happens rather than what other Pokémon games do.
Getting an egg
Leave two compatible Pokémon at the Day Care on Route 34 and they may produce an egg.
Baby Pokemon
Baby Pokemon (Wynaut, Azurill, Budew, Chingling, Bonsly, Mime Jr., Happiny, Munchlax, Mantyke) breed normally - no Incense needed.
Setting: P_INCENSE_BREEDING = GEN_LATEST (Gen 9)
Nidoran and Volbeat with Ditto
Nidoran-M and Volbeat bred with Ditto can produce female offspring (Nidoran-F / Illumise).
Setting: P_NIDORAN_M_DITTO_BREED = GEN_LATEST (Gen 9)
Teaching egg moves in the Day Care
A Pokemon holding a Mirror Herb in the Day Care learns Egg Moves from the other Pokemon, regardless of species.
Setting: P_EGG_MOVE_TRANSFER = GEN_LATEST (Gen 9)
Egg groups
Gen 8's egg group additions are applied.
Setting: P_UPDATED_EGG_GROUPS = GEN_LATEST (Gen 9)
Vivillon patterns
The Scatterbug, Spewpa and Vivillon line always breeds into the Fancy pattern.
Setting: P_SCATTERBUG_LINE_FORM_BREED = SPECIES_SCATTERBUG_FANCY
What the baby inherits
What the hatchling gets from its parents, and how to influence it.
Poke Ball
The egg inherits a parent's Poke Ball; the father can pass it down too, as long as he's the same species as the mother.
Setting: P_BALL_INHERITING = GEN_LATEST (Gen 9)
TM moves
Fathers do not pass down TM moves.
Setting: P_TM_INHERITANCE = GEN_LATEST (Gen 9)
Egg moves
Either parent can pass down Egg Moves.
Setting: P_MOTHER_EGG_MOVE_INHERITANCE = GEN_LATEST (Gen 9)
Nature
A parent holding an Everstone always passes down its Nature.
Setting: P_NATURE_INHERITANCE = GEN_LATEST (Gen 9)
Ability
80% chance to pass down a regular Ability, 60% for a Hidden Ability. Either parent can pass it, including when bred with Ditto.
Setting: P_ABILITY_INHERITANCE = GEN_LATEST (Gen 9)
IVs
3 of the baby's six IVs are inherited at random from its parents; the rest are random. A parent holding a Destiny Knot raises that to 5.
Setting: src/daycare.c = InheritIVs
Power items
A parent holding a Power item (Power Weight, Power Bracer and so on) always passes down that item's IV, using up one of the inherited slots. Both parents holding one means one of them is picked at random.
Setting: src/daycare.c = HOLD_EFFECT_POWER_ITEM
Hatching
Eggs hatch as you walk. Nothing else is needed.
What level they hatch at
Eggs hatch at level 1.
Setting: P_EGG_HATCH_LEVEL = GEN_LATEST (Gen 9)
How long an egg takes
Egg cycles use the original (longer) step counts.
Setting: P_EGG_CYCLE_LENGTH = GEN_3 (Gen 3)
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