Credits to Schguki, Potobc, Aradi, GReinoso96

Item Combinations

Extract itemPreData from loc\arc\resident.arc\table, can be expanded.
Each entry is 24 bytes long
Combination/Sorting ID (+0, byte)
First Item position ID (+2, byte)
Second Item position ID (+6, byte)
Resulting Item ID (+10, byte)
Success Probability (+14, byte), can be any value but only 100/95/90/75/65/55% are used since those are the cutoffs for the combo books
Item Amount (+15, byte), refers to a table in itemPreTypeData with 00 being the first one
A few combos have some values after but I'm not sure what those are
Item position ID refers to the table in itemData, it’s a position ID and not an item ID as changing the value of the ID in itemData does not affect the combinations that use it

Item Amount

Extract itemPreTypeData from loc\arc\resident.arc\table. Can be expanded.
Each entry is 8 bytes long
Minimum Amount (+0, byte)
Probability of each amount (+1-6, byte), should always add up to 100% (haven't tested what happens if its not 100%)
Maximum Amount (+7, byte)
Due to how you only have 6 probability values you cannot make a combination have a minimum and maximum amount with a difference > 6 (probably, not tested)
Tables 05, 07, and 08 aren't used

Items are defined in:

table/itemData.itm

Actual table\itemData.itm file is located inside resident.arc inside NativeNX\loc\arc for MHGU, and NativeNX\arc for MHXX Switch ver.

The header is 8 bytes long, the first 4 bytes are always:

66 66 AA 41

The next 4 bytes correspond to the total number of entries, including the first dummy entry:

AF 0B 00 00 = 2.991 Entries

Increase this value when you add new entries.

The following is a rundown of values for each entry, 44 bytes for each entry, in order: