Why are these resistors between different nodes assumed to be parallel
In the solution, the first step that is done is to combine \$R1, R3, R6\$
into an equivalent resistance \$Ra\$.

If you are trying to find the effective Thevenin impedance looking into nodes A and B  whilst ignoring the load resistor RL, then that is exactly what you do. Trying to find an effective resistance means that voltage sources are short circuited (or set to 0 volts) and current sources are made open-circuit: -