how are the sum of squares computed for anova

I've fit my multi-variable data to a linear model using the fitlm function from the Statistics and Machine Learning Toolbox. The output is a linear model which I then pass as an argument to the anova function. The output is a table listing a SumSq for each term in my model and the resulting F statistic and pValue.

       >> anova(mdl_stp)
    > In classreg.regr.CompactLinearModel>getTermSS1 (line 1336)
      In classreg.regr.CompactLinearModel/componentanova (line 717)
      In classreg.regr.CompactLinearModel/anova (line 311) 
    Warning: Matrix is close to singular or badly scaled. Results may be inaccurate. RCOND
    =  3.145894e-21. 
    > In classreg.regr.CompactLinearModel>getTermSS1 (line 1336)
      In classreg.regr.CompactLinearModel/componentanova (line 717)
      In classreg.regr.CompactLinearModel/anova (line 311) 
    Warning: Matrix is close to singular or badly scaled. Results may be inaccurate. RCOND
    =  1.500654e-17. 

    ans =

      8×5 table

                 SumSq      DF      MeanSq       F         pValue  
                 ______    _____    _______    ______    __________

        x        156.94        1     156.94     164.8    1.5552e-37
        y        183.63        1     183.63    192.82    1.3846e-43
        q        132.08        1     132.08    138.69    6.9855e-32
        x:y      19.285        1     19.285    20.251    6.8386e-06
        x^2      38.509        1     38.509    40.438    2.0849e-10
        y^2      37.174        1     37.174    39.036    4.2667e-10
        q^2      11.728        1     11.728    12.315    0.00045057
        Error     15077    15832    0.95229

My question is how are the SumSq terms computed? I could not find any of the original data in the model, so I don't think additional fits with and without terms are performed. What then makes up the contributions of each term?