what the means of select 123&789

"&" is a binary AND operator: it combines bits in the operand, so if the bit is one in both operands, than the result is the same bit one - other wise it is zero.
So, for each bit:

Op1  &   Op2   Result
 0        0       0
 0        1       0
 1        0       0
 1        1       1

It's kind hard to see from your example what is going on, because all your numbers are in decimal - which doesn't relate "nicely" to the actual bits. But...if you translate them to binary values first it's more obvious:

Your command       Op1             Op2            Result
select 1&1          01              01               1
select 1&2          01              10               0
select 1&3          01              11               1
select 1&4         001             100               0
select 123&1   1111011         0000001               1
select 123&7   1111011         0000111              11
etc.