How else, besides conjugation, can horizontal gene transfer occur?
There is transformation and transduction.
During transformation, a bacterium takes up DNA that it encounters in its local environment. The outer membranes have proteins that bind DNA and transfer it into the cell.
In transduction, the transfer is mediated by a bacteriophage, some of which don’t kill the host cell but instead integrate into the host bacterium’s chromosome, becoming a prophage. Eventually, the prophage gets excised and the new virus particles will be constructed around the replicated virus genome. The bacterial chromosome is also broken down. Because of this, one of the small chromosome fragments/ bacteriophages might be packaged into a virus particle. The DNA is still infective.