can someone explain how to create new features using feature interactions?

There is this notebook solving housing prices.

https://www.kaggle.com/code/jesucristo/1-house-prices-solution-top-1/notebook?scriptVersionId=12846740

and it had this bit of code, can anyone explain the how addition and multiplication and weighs work?

features['YrBltAndRemod']=features['YearBuilt']+features['YearRemodAdd']

features['TotalSF']=features['TotalBsmtSF'] + features['1stFlrSF'] + features['2ndFlrSF']

features['Total_sqr_footage'] = (features['BsmtFinSF1'] + features['BsmtFinSF2'] +
                                 features['1stFlrSF'] + features['2ndFlrSF'])

features['Total_Bathrooms'] = (features['FullBath'] + (0.5 * features['HalfBath']) +
                               features['BsmtFullBath'] + (0.5 * features['BsmtHalfBath']))

features['Total_porch_sf'] = (features['OpenPorchSF'] + features['3SsnPorch'] +
                              features['EnclosedPorch'] + features['ScreenPorch'] +
                              features['WoodDeckSF'])