There are many useful tools supplied in the QGIS Processing Toolbox. However, there are also some useful processing tools that don't come out of the box, particularly in relation to hydrology.
SAGA GIS consists of nearly 800 processing tools across 90 categories. It used to be included in QGIS, but as of QGIS 3.30, needs to be installed separately. Even before QGIS 3.30, some of the tools were missing due to version incompatibilities. There are videos and instructions on how to install this on the internet, but they may not be up to date with the latest changes to both SAGA and QGIS.
The cleanest way to install is to grab the zip file from the SAGA downloads page, and extract to a folder eg
Once installed, in QGIS, you will then need to go to Plugins→Manage and Install Plugins. Search for 'saga'. Find “Processing Saga NextGen Provider” and then Install Plugin.
Finally, you need to go to Settings→Options and then Processing, and under Providers→SAGANG→SAGA Folder, set the path to the folder that you installed it to above.
You should now be set up with additional SAGA Next Gen tools in the Processing Toolbox.
You can also install SAGA using the OSGEO4W Setup tool, but I hit problems with this which I couldn't resolve.
Whitebox Tools consists of around 475 processing tools across 17 categories. It is open source, but accepts donations and has paid extensions. The tools are generally very fast and efficient.
Whitebox Tools has a video explaining how to do this, as well as how to install the QGIS plugin needed to integrate this into QGIS.
Similar to SAGA, the easiest way is to download the zip file for WhiteboxTools Open Core (WbT) from the Whitebox Tools website and extract to a folder eg
Once installed, in QGIS, you will then need to go to Plugins→Manage and Install Plugins. Search for 'whitebox'. Find “WhiteboxTools for QGIS” and then Install Plugin.
Finally, you need to go to Settings→Options and then Processing, and under Providers→WhiteboxTools→WhiteboxTools executable, set the path to the executable file eg
You should now be set up with Whitebox Tools in the Processing Toolbox.