If i render as open EXR but not Multi layer will i still have access to all the passes and will be able to recombine them with my layers in compositor?
OpenEXR, by itself, is a loss-less floating-point image format, designed (by ILM) for use as an intermediate file in a CG rendering pipeline.
MultiLayer is a Blender Foundation(!) extension to that format … since accepted by the industry … that stores multiple layers of information into a single file, using the same floating-point format for each layer. (Previously, it was necessary to use multiple files, with all the inherent problem of making sure they actually correspond to one another. Ick.)
“‘Image file’ formats” are designed for image display, possibly on cheap and under-powered equipment. They do not “preserve the entire numeric data-set that the render has produced, exactly as it was produced, file-size b’damned,” as MultiLayer OpenEXR does.