I present results of fully self-consistent synthetic dust polarisation maps created with POLARIS of simulated molecular clouds (MCs) within the SILCC-Zoom project. The MCs form in simulations of a 3-phase ISM which include its detailed chemical and dynamical evolution. First, I assess the reliability of polarisation observations from e.g. Planck and BlastPol. Next, discussing in detail the complex structure of MCs revealing both sheets and filaments, I will show how projection effects affect the observed orientation of magnetic fields. Furthermore, I will dicsuss the alignment efficiency of dust grains and show that polarisation observations of MCs mainly probe their dense parts up to AV > 1 (>= 1000 cm^(-3)). In addition, I combine my results with analytical considerations to shed light on the dynamical importance of magnetic fields. I will show that magnetic fields become perpendicular to (column) density structures above N = 10^(21-22) cm^(-2) and n ~ 1000 cm^(-3) and discuss how this orientation flip relates to the question where and when gravity starts to dominate over magnetic fields. This will give a holistic view of how magnetic fields affect both the dynamics and chemical evolution of molecular clouds. Finally, I will shed light on our latest results about which effects influence the wavelength dependence of the polarisation degree. Specifically, I will discuss the potential origin of V-shaped polarisation spectra in MCs.
