Make or take a phone call. There's a speaker icon on the screen. Tap it. (I don't know of a way to make the speaker the default, unless you use voice control to answer a call [it's in Settings in the Controls section].)
There is no speakerphone switch that works for everything. Each application has its own way of turning on the speakerphone. As stated above, it's usually a speaker icon. You can use various tasker programs to make the speakerphone the default.