How to create supports for the parts hanging above 45 degrees? Do you have any specific process recommendations to achieve support for the parts hanging above 45 degrees, using open source software?
PrusaSlicer has support enforcers you can place on areas that need support.
See this video: Prusa Slicer Support Enforcers.
Automatic supports
There are automatic supports, in the Supports dropdown menu, select Everywhere:

Click Yes in the resulting dialog:

Click on the Slice icon in the bottom left:

You will end up with a lot of supports:

However, this method generally results in too much support...
Custom Suport Enforcers
So turn off Supports in the drop down menu, select None:

Now, in the Print Settings tab, under Support Material, disable Auto Generated Supports and enable Generate Support Material:

Then right click on the model and select Box from the Add Support Enforcer menu item:

You can move by clicking the Move button in the left hand palette,

resize by clicking the Resize button

and re-shape the Box as necessary, to support the difficult overhanging parts parts only. Here we can see three boxes have been added - pale blue for the previously added boxes and green for the current box:

And a fourth final Box for the tail:

Now, when you hit the Slice button, there will be much less support structure than when using Automatic supports:

Parts of the model that the slicer thinks still needs support will be highlighted in dark blue (such as the elbow and back of the head):

Add a cylinder support enforcer:

Resize and re-shape as before and move into position below the elbow:

Adding a second cylinder as a support enforcer

Upon hitting Slice:

Now in Print Settings - Support Material - Pattern change it from Rectilinear to Rectilinear Grid:

For prints with curves and details Rectilinear Grid works better, than Rectilinear (which is fine for supporting a plain cube in the air). It is easier to break the support off the print.
Now save your hard work as an AMF file:

This file maintains all of the support enforcers so that they can be modified if the actual print needs some adjustments - without having to re-add all of the support enforcers all over again.