They're in the DXF code 12 and 13 entries in entity data -- see >this<. -- or the StartTangent and EndTangent VLA properties, or in the (getpropertyvalue)/(setpropertyvalue) world, the StartFitTangent and EndFitTangent properties.  You can play with them in the Properties palette, too.

With the POINT command, with Snap turned on so they are all at grid locations.  They are there in the image only to illustrate -- it is not necessary to have them there if you can visually pop to positions in that kind of regular relationship.  In the case of the relationship in my image, the relative position of each location is like a Knight's move in chess, but you'll need to experiment to decide whether that makes a shape enough like what you want.

in the CAD / BIM Blocks library and / or test the command SPLINE on your computer.

In the old AutoCAD version you could do this after but now I haven't find the way to "catch" those tangents so I add multiple in line points and then trim the spline were it normaly begins and ends.

Any informations about start and end tangents ?

I had previously posted a discussion about that with an AutoCAD file but got no answers.