IR light has a wavelength that is the same size or bigger than dust grains so it can penetrate dust clouds. Grains are made of cores and mantles. The surfaces of dust grains may be ice or tarry mixes of molecules. Its on the surface of dust grains that molecules form and then get ejected. That is one reason why they are so important.
Dust can only form at temperatures below a few thousand degrees.
Dense grains probably from in the winds of old giant stars. Cool old stars can lose a millionth of a solar mass in year in a wind. As this stuff expands and cools dust condenses. The ices probably form later in the depths of molecular clouds. Star Formation Star Formation is a multistage process which we are still struggling to understand.