Why is there a 2 in front of some of these passive component parts?

Well, \$R_B\$ is defined as 1 MΩ and so \$2R_B\$ is 2 MΩ: -

It means whatever value you choose for \$R_\mathrm B\$, use \$2\times R_\mathrm B\$ at that position. Eg \$R_\mathrm B\$ is \$1\ \mathrm{M\Omega}\$, \$2R_\mathrm B\$ is \$2\ \mathrm{M\Omega}\$.

It means that the 2R_A resistor values are twice those of the R_A values and similarly with the R_B values.

If R_A is 1 kΩ then the 2R_A value is 2 kΩ.