How can countries without extradition treaties arrest people?

You can arrest people through alternative measures such as repatriation, persuasion and prosecution. Repatriation is an informal way of international assistance, which means that in order to institute criminal proceedings or execute criminal punishment against fugitives, countries wishing to institute criminal proceedings or execute criminal punishment provide their criminal clues to the countries where the fugitive suspects or criminals are located, and the latter will forcibly repatriate the criminals to third countries or fleeing countries according to the relevant provisions of their own immigration laws.