The Large Hadron Collider is the largest and most powerful accelerator in the world. Proton bunches are accelerated to 6.5 TeV inside the LHC ring in opposite directions and are then collided at four different points around the ring with an energy of 13 TeV. The ATLAS detector is a general purpose particle physics detector built around one of the collision points and is used to study the products of the high energy proton-proton collisions. The identification of jets containing b-hadrons (b-jets) is an important part of many physics analyses, including those presented in this thesis. The calibration of the b-jet identification algorithms is performed with data. The first paper of this thesis describes the method to calibrate the probability of a light jet to be mistakenly identified as a b-jet. The second paper presents a search for new phenomena in events with top quark pairs and large missing transverse energy. No evidence for physics beyond the Standard Model is found and the analysis is instead used to exclude various extensions of the Standard Model. The third paper describes the search for Higgs boson pair production in a final state with two photons and two b-jets and the fourth paper a novel Effective Field Theory interpretation of this search. Finally, a search for models with beyond the Standard Model Higgs bosons, X and S, in a final state with two photons and two b-jets is presented. The signal targeted by this search is the resonant production of an X boson that decays to an S and the Standard Model Higgs boson H. This search is under development and the results have not yet been published. All papers included in this thesis use data recorded by the ATLAS detector between 2015 and 2018, during the Run 2 of the Large Hadron Collider.