Searches for new phenomena in final states with $3^{\mathrm{rd}}$ generation quarks using the ATLAS detector

Many theories beyond the Standard Model predict new phenomena, such as $Z'$ and vector-like quarks, in final states containing bottom- or top-quarks. It is challenging to reconstruct and identify the decay products and model the major backgrounds. Nevertheless, such final states offer great potential to reduce the Standard Model backgrounds due to their characteristic decay signature. The latest search in the third-generation quark final states using the full Run-2 proton-proton collision data collected by the ATLAS experiment are presented. Particularly, the recent results of di-bjet and top-antitop resonance searches and dark matter produced in association with a top-quark are discussed in this proceedings. The associated improvements in $b$-quark and top-quark identification techniques are also highlighted.