I don't know what you mean with folded in this circuit. This is considered a "sub-bandgap voltage reference" since it is below the usual 1.25V bandgap voltage. Because of that, it can also operate at power supplies <1V (which can be considered "low voltage").
The short version of expression is that you add a PTAT current to a CTAT current to get a zero to absolute temperature (ZAT) current.
PTAT comes from \[\Delta\] VEB /R1 = VT ln / R1.
CTAT voltage VEB is divided by (R2 + (R4||R5) to give a CTAT current.
Both currents are added in IMP4 (ZTAT current).
IMP4 is copied into IMP6 and flows in R3 to produce a ZTAT output voltage as given by the expression you have derived.
The short version of expression is that you add a PTAT current to a CTAT current to get a zero to absolute temperature (ZAT) current.
PTAT comes from \[\Delta\] VEB /R1 = VT ln / R1.
CTAT voltage VEB is divided by (R2 + (R4||R5) to give a CTAT current.
Both currents are added in IMP4 (ZTAT current).
IMP4 is copied into IMP6 and flows in R3 to produce a ZTAT output voltage as given by the expression you have derived.