Why build phase is top – down & connect phase is bottom – up?

The connect phase is intended to be used for making TLM connections between components, which is why it occur after build phase. It work bottom-up so that its got the correct implementation all the way up the design hierarchy, if worked top-down this would be not possible

Which uvm phase is top - down , bottom – up & parallel?

Only build phase is a top-down & other phases are bottom-up except run phase which is parallel. The build phase works top-down since the testbench hierarchy may be configure so we need to build the branches before leafs