I downloaded Sentinel2 from EoBrowser - bands 1, 2, 3, 4, 8.
But I don´t know, in which order should I put those bands into Composite Bands so that in output raster will be 123 as true colors and 432/843 as false colors?
yes, but in which order should I place them, when I want to have just one raster with possibility choose between true and false colors by changing bands? or it doesn´t matter?
Sure, just throw them all together in the tool then choose/manipulate the band combinations as you need afterwards. Or forget about the composite bands GP tool and just use the image analysis window - as this video demonstrates (ensure you turn your sound off as the music is awful) Composite Bands (Data Management) - ArcGIS - YouTube