The simplified schematic of the first stage is as follows:
simulate this circuit – Schematic created using CircuitLab
The upper block is a voltage divider to form a virtual ground which is essential for single-supply opamp amplifiers so that the positive input terminal of the opamp to be greater than GND level (Although it's output can be anything between GND and +9VDC, set at half of the supply voltage, 4.5VDC, for symmetrical clipping). Note that this virtual ground point should be go to ground at AC analysis. That's why a large capacitor (C1) is placed across the output. For virtual grounding, \$in_p\$ terminal of the opamp should be connected to that virtual ground point with a random value resistor (which is R4=100k here and doesn't affect the gain).
The second block is a non-inverting amplifier. Now remember a conventional non-inverting amplifier:
Lower end of \$R_g\$ is connected to ground. But this scheme is valid for dual-supply (i.e. supplied both positive and negative sources, such as ±12V) opamps. But ours has a single-supply config, so the lower end of \$R_g\$ should be tied to the "virtual ground". Look carefully, you'll see that R3 (which is the equivalent of \$R_g\$ in the above scheme) is connected to virtual ground point.
Finally, gain is \$K_v = 1 + R_5/R_3 = 1 + 1M/100k = 11\$.
Note that the coupling (DC blocking) cap is not used at the output of the amplifier stage. Because the following buffer stages need their \$in_p\$ terminals to be at virtual ground, so the DC level of the amplifier's output can be used as a virtual ground for folowing buffers.
Following three stages are simple buffers. The 5k1 resistors at the outputs of these three buffers are just pull-down resistors and do not affect the output impedances at all. These buffers have very small output impedances, so if you want them to be 300-600Ohms, just place that value resistor in series (5k1 resistors are optional. I didn't show, but coupling caps are essential):
Buffers can be used either line, instrument or mic level. You can just modify the first amplifier stage according to your needs by adjusting R5 and/or R3 resistors.