Yes, Mi A2 is made of Aluminium Alloy.
No. Mi A2 has USB Type C port and it also comes with USB Type C to 3.5 mm headphone adapter in the box. So, there will not be any problem to use Mi A2 with your 3.5 mm headphones.
Yes, Mi A2 does have the fingerprint scanner. It also has face unlock support.
Yes, you can record 4K videos via Mi A2. It does support 4K video recording.
The Mi A2 continues Xiaomi's now-familiar tradition of offering excellent value. The upgrades from last year make it an enticing option in this category, and the camera, in particular, is stunning.
The Xiaomi Mi A1 was an incredible value proposition in its market segment, the Mi A2 falls somewhat short. It takes away key features like storage expansion and universal headphone connectivity, plus it misses the mark in crucial areas like display and battery life. Affordable as it may be, it wouldn't be our default choice in the midrange quite like the A1 was.