Junction temperature calculation
An estimation is possible with:
$$T_j = P \times R_{th} - T_a$$

There is a mistake in your first formula. It should be this: -
$$T_j = P\times R_{th} + T_a$$
In other words, if the ambient temperature rises then so does your junction temperature.

Can someone explain to me how I can calculate the power loss of power
semiconductors without knowing the junction temperature?

The junction temperature doesn't need to be known because power dissipated is volts x amps or amps squared x resistance or volts squared divided by resistance.