Here the word "They" refers to all of the believers, but it was probably one of the apostles who spoke these words. Alternate translation: "The believers prayed together and one of the apostles said" (See: [[rc://en/ta/man/translate/figs-explicit]])
Here the word "hearts" refers to the thoughts and motives. Alternate translation: "You, Lord, know the thoughts and motives of everyone" (See: [[rc://en/ta/man/translate/figs-metonymy]])