Are 2 x 12 x 10 boards exactly 2 inches thick or what?

Once upon a time they were 2" thick (and all dimensioned lumber was the actual dimensions). Most of that lumber was rough cut (not smooth).

In general, people wanted smooth lumber. So a 2" thick rough cut board was machined into a 1 3/4" smooth board.

This applies to all dimensioned lumber today. The stated dimensions are the size of the rough lumber fed into the machines to smooth it to a smaller sized smooth board or framing member.

You know it would take less time to measure than to login and post about it.

Most 2 by are not two inches thick. If you sandwich them together it will be close to 3.5

Yes, it would then be 4x6x10 or 4x12x5 depending on which way it was cut.