The numbers go like this.
The first number is how "good" it is. 0 being the worst 5 being the best, sometimes samsung makes a 7. Higher number phones cost more and are usually just better.
The second number is how new it is. 6 is the current generation.
So the a12 and a16 are the same level of "goodness" for their year. But the a16 is significantly newer. In the uk, the lowest number you can usually buy is a 1. 0 phones are usually sold online only or in other countries with a lower general income.
Updates work like this.
When a phone is first released, samsung says how many years of updates it will get. Say a phone comes out and has 2 years of updates. You buy it 9 months after it comes out. Your phone has 1 year 3 months of updates before they stop.
From a little research, I think the a12 stopped getting updates in January 2025. It got 4 years of updates, came out in 2021. The cheaper A series phones also update quarterly, so 4 times per year. More expensive A series phones update each month
The S series phones go up each year. So s22, then s23, then s24, then s25.
The A series is a budget line up so it's totally separate from the naming scheme of the S series. I mentioned how they work earlier in my comment.
For the things I didn't answer, I don't know enough to confidently give you an answer.
I've never owned an a12 or a16 so can't tell you the difference between them, except that the a16 is significantly newer
