Minecraft wiki entry for Fluid, associated text: If lava flows vertically into a water source block or flowing water, the water source block turns into stone.
If lava flows horizontally into water, the lava turns into cobblestone.
If water flows horizontally into flowing lava, the lava turns into cobblestone. Prior to Java Edition 1.16 snapshot 20w13a, a hiss sound and puff of steam effect occurred, but the water and lava remained unchanged.
If water flows vertically into flowing lava, either cobblestone or nothing may result.
If water flows into a lava source block, the lava source block turns into obsidian. Because of this, unlike with cobblestone, obsidian is not renewable with this method.
If water falling vertically touches a lava source block on any side, the lava turns into obsidian—even if the water would not otherwise run into the lava.
If lava flows over soul soil next to blue ice, the lava turns into basalt.