3GPP 38.300 v19.3.0 — the document's own text
4.7.4.5 OTA timing synchronization
Taught in 4. What NR actually is, in the shape its own document gives it (TSG RAN — the group that specifies the radio, overview).
An IAB-DU is subject to the same downlink timing alignment of a gNB. The IAB-DU may use the received downlink signal from a parent as a reference to control its downlink timing using TA in conjunction with an additional Tdelta parameter received by the collocated IAB-MT from the parent via MAC-CE.