If miRNA and mRNA are made of the same nucleotides, then why are miRNA not translated like mRNA?
Is it due to miRNA lack of start codon? Or lack of promoters?
Has it got anything to do with RISC?
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As you've noted, different RNA molecules are treated differently in (eukaryotic) cells (other common RNAs are tRNAs and rRNAs). Factors influencing this include the RNA sequence involving both sequence motifs and non-canonical secondary structures, chemical modifications such as caps and poly-A tails on mRNAs, interactions with protein complexes (such as transcription complexes or ribosomal complexes), and localization within the cell.
Regarding translation of RNAs, a useful review is "Principles of Translational Control: An Overview" Hershey et al 2012
Germane to your question, initiation of translation is complex and specific. The authors note
"A conspicuous feature of eukaryotic protein synthesis is the fact that mRNAs are translated in the cytoplasm, making translation uncoupled from transcription. Mature eukaryotic mRNAs possess a m7G-cap at their 5′-terminus and, in most cases, a poly (A) tail at their 3′-terminus. ... the pathway and mechanism for the formation of 40S and 80S initiation complexes differ substantially from those in bacteria. For example, a large number of initiation factors (at least 12) promote the binding of the mRNA and initiator methionyl-tRNAi (Met-tRNAi)—which is not formylated—to the 40S ribosomal subunit. "
Regarding miRNAs, a useful review is "Overview of MicroRNA Biogenesis, Mechanisms of Actions, and Circulation" O'Brien et al 2018
In particular, Fig. 1 is a useful cartoon diagram of the different known pathways producing and processing miRNAs and the subcellular locations of these events.
Later on, they also discuss RISC as one of the mechanisms of miRNA-based gene regulation.
As detailed knowledge of subcellular processes has exploded in the last two decades, it's always good to try to use the most up-to-date textbooks or reviews you can find.
