What is the difference between the two? i have 19.5.435 Py3.9, what does py37 stand for?

It denotes the version of python that that build of Houdini utilizes. Py39 is Python 3.9, Py37 in Python 3.7. The difference is mainly for studios that require a specific version.

The default version refers to py39 ? There are two files of every build on the website, default and py37.
Also through the launcher, same thing.
First time I installed I didn't notice and I got the py39 by default. Just wanted to verify it

The "default" version matches the VFX Reference Platform [vfxplatform.com]'s Python version for the year the Houdini version is released and any secondary supported versions are explicitly labeled as py#. In the case of Houdini 19.5, it is Python 3.9 as it was release during CY2022 and the secondary version is py37, which was the Python version of the previous release.

Similarly, Houdini 19.0's default is 3.7 (CY2021) and has an altertate version of py2(2.7)