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It would take too many images to show how every bit of this is configured, but I'll give a general explanation instead. Even that's gonna take a while; if anyone has specific questions about how this works, ask and I'll try to answer!

the "input" is iron plates in smartchest near the top-left of the pic; I set it up to fill the smart chest with 999 iron plates from another chest, so it currently counts up from 0 to 999 at the inserters' speed. That's not a limitation, though; the display updating is effectively instantaneous as the chest contents change, regardless of how they change.

The input chest links to a pair of arithmetic combinators; the first divides by a constant 10, and the second takes that output and multiplies it by -10. (division seems to round down, so 10/10 = 1, but so does 11/10, and 19/10, etc)The result of the division is passed down and used as the input value for the next higher digit's logic. The result of the multiplication is joined with the original input value, which effectively subtracts it - so, ex, if the input was 99, divided by 10 gives 9, multiplied by -10 gives -90, then 99 + -90 = 9. That gives us the digit, and is done by just those two arithmetic combinators at the left of each block of logic. That combined value is fed as input to 10 decider combinators, which test if the value is equal to 0, 1, 2, etc. through 9, each outputting a 1 if true on the corresponding numbered signal line. These outputs are all combined, giving me one red cable network with correct signal, 0-9, set to "1" and the rest "0".

That output feeds the input of the 3rd stage, where a series of decider combinators convert those values into segment values. That part is a bit tricky, so I'll walk through an example.

I numbered the segments on the display, starting from the top horizontal one as "0", then the top-left, top-right, middle, etc. For each segment, I basically list out the numbers for which it is on and off; the top segment is off when displaying a "1" or a "4", and on for 2, 3, 5, 6, 7, 8, and 9. So for it, I simply used 2 DCs, testing input signals 1 and 4 and outputting on signal 0 if they were set. That output feeds to the lights, and two lights making segment 0 are set to be on if the value of signal 0 was "2." This is basically an "and" gate. Other segments are more ... bothersome; segment 4, in the bottom-left, is off for 1, 3, 4, 5, 7, and 9, and on for 0, 2, 6, and 8, so in that case I tested for the 4 on values instead of the 6 off values, outputing on signal 4 if the input signal was true, which means the lights for that segment are set to be on if signal 4 is >0. The rest are done basically the same way.

Each block below the first, instead of using a chest for input, takes input directly from the division combinator of the block above it. The system could be scaled up to as many digits as you wanted.