https://doc-snapshots.qt.io/qt5-5.15/wasm.html explains how to install the Emscripten SDK and gives a copy-paste-able example on how to activate it. The problem is that the copy-paste-able activation command on the Qt 5.15 docs page installs and activates the SDK version for Qt 5.13.
An additional trap is that Qt 5.15 does not seem to work with the fastcomp variant of enscripten, so, that prefix needs to be omitted, as-well.

Users might blindly copy that line and end up in trouble later. (Happened to me more than once)