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The link rate adapts based on the SNR. 

From SNR, Eb / N0  is got by converting the logarithmic decibel scale SNR to linear power scale, and normalizing it. 

Then Efficiency = log2(1 + Eb/N0), where Eb/No is signal over noise.

Then a table is looked up to get CQI from efficiency.  These are the tables referenced in Based on the CQI index the equivalent modulation and code rate is decided. 

And then based on MCS. the number of information bits is got from the Transport Block Size Determination calculations in standard 38.214 -

For additional understanding open the 5G technology library manual

and see example 4.3. A user would get an idea how how MCS is chosen looking at the results table of that example.