An interesting observation about phototropism is the tree / limbs are actually growing away from the dark not as we think towards the light. Its kinda counta intuitive but I'll try ma best to explain.
An auxin called indole-3-acetic acid, or IAA does a lot here, its a plant cell growth/regulator hormone.
IAA is mostly manufactured at the apical meristem, the very top of the plant. From there it travels down the stems, to the roots. Moving down the stem, it starts to get a bit darker, as you would expect. But IAA likes the dark and not the light so it triggers cell elongation on the dark side of a limb or trunk. On the sunny side of limbs IAA breaks down in light and so you get limb/trunk cell length shorting.
This causes the limb to grow/lean away from the dark, or towards the light. This is phototropism why plants appear to grow towards the light. They don’t. They're growing away from the dark.