I’m yet unsure whether to study Enviro. Eng. , Architecture, Economics, or even political science! So does declaring my major affects the chance of acceptance?
Depends if the college admits everyone in the same pool, or has separate pools depending on major. In general, Engineering majors are more selective than arts and sciences (econ, poli sci, etc.).
As in almost all cases it depends on the school. Are each in different colleges? Do you have to apply to the College vs. the U? Do they get admitted differently? Some colleges admit by major (like the UCs).