Did the Buddha chant some mantra or holy word throughout his meditation? Or he was just silent to realize what is coming through him to interpret?
more like contemplating rather chanting.
origination or dependent arising, Pratītyasamutpāda. He told Ananda he would contemplate, recite Pratītyasamutpāda when he was alone.
During his developmental years (before he became Enlightened) he might have. This is because mantras are a way for you to match vibration with a particular deity and thus receive their assistance.
The Buddha gave many such great mantras including the Zhunti mantra but a Buddha has no reason to use them because the Buddhas are all connected simultaneously through their uncovered dharmakaya/Tathagatagarba/Buddha-nature.