Final answer:
In mainland Southeast Asia, population concentrations are found along coastlines, rivers, and fertile plains due to the amenities and resources these areas provide, such as for agriculture and trade.
Step-by-step explanation:
In mainland Southeast Asia, the population tends to be concentrated in areas with access to water and fertile land. This means that many people live along coastlines, near rivers, lakes, and the oceans. Historically, these areas provided the necessary resources for agriculture and trade, thereby supporting larger populations. For instance, the river valleys and deltas in countries like Vietnam, with the Mekong Delta, and Thailand, with the Chao Phraya River Valley, are densely populated. Additionally, areas with a high food-producing capacity, such as the fertile plains, support larger populations. This pattern of settlement aligns with global patterns where large population clusters are often located in regions with high urbanization or strong agricultural output.