District 1: touristy hub
District 5: Chinatown area
District 7: newer area, many expats, a 'Vivocity' mall.
You can walk within districts 1, 3 and 5. You might need to take transport to district 7.
Last time I was there the Saigon MRT was still under construction. You might be able to take MRT directly to district 7 soon.
https://hochiminhcitymetro.com/maps/