Type in any field and the others update. Numbers of any size are exact (BigInt), and a leading - makes them negative.
Prefixes are accepted where they match: 0b in Binary, 0o in Octal, 0x in Hex. The Decimal field takes any of them, so you can paste 0xFF there. Spaces and _ are ignored, e.g. 1111_0000 or 1 000 000.
Group digits only changes how numbers are shown; the copy buttons copy plain digits.
Two's complement shows the bit pattern a value has in an 8/16/32/64-bit integer. A positive value above the signed maximum fits only as unsigned.
Shortcut: Ctrl/⌘+Enter copies the field you are in (Decimal otherwise).