We live in the era of precision cosmology, upcoming optical and radio surveys will probe high redshifts, large volumes of the sky, and non-linear scales with high precision. As a result, future surveys open up the opportunity to test cosmological and gravity models in a wider range of distances and scales. In order to use the potential of the new precise data, we need to include a more precise modeling in our model constraints and take into account the effects that have been neglected so far. The precision of the new data will be sufficient to detect some of these effects, like lensing magnification. If they are taken into account, then they will help to constrain cosmological models, otherwise, they will show up as systematic biases in the results. In this thesis, we have studied some of these effects.