On Thingiverse you can download and share numerous models and apps.
There is also the so-called "Customizer", with which you can construct and vary 3D models with corresponding templates and parameters.
To generate trees, I use this template.
There you may not click on "Download all Files", but on "Open in Customizer". Then you can define with several parameters how many ramifications there are, for example, how long they are and what shape they have. Most of the default settings are already usable, but should be slightly varied if you want to generate several trees.
Finally, you add your creation to the queue. It takes a few minutes until the *.stl file is generated.
How the *.stl file is processed further is explained in part 2 of the series. Theoretically, you can print the file directly if your print area is large enough. If you have any questions about Thingiverse and its customizers, please leave comments.