In the following I'll try to address the topic of my comment which you've quoted and which I must correct here.
Whether what was drawn has a soul is the key criteria for prohibition
The point is that being alive or having a kind of life inside seems not equal to having a soul.
You may read in Sahih al-Bukhari:
While I was with Ibn 'Abbas a man came and said, "O father of 'Abbas! My sustenance is from my manual profession and I make these pictures." Ibn 'Abbas said, "I will tell you only what I heard from Allah's Messenger (ﷺ) . I heard him saying, 'Whoever makes a picture will be punished by Allah till he puts life in it, and he will never be able to put life in it.'
مَنْ صَوَّرَ صُورَةً فَإِنَّ اللَّهَ مُعَذِّبُهُ، حَتَّى يَنْفُخَ فِيهَا الرُّوحَ، وَلَيْسَ بِنَافِخٍ فِيهَا أَبَدًا
" Hearing this, that man heaved a sigh and his face turned pale.
Ibn 'Abbas said to him, "What a pity! If you insist on making pictures I advise you to make pictures of trees and any other unanimated objects."
فَقَالَ وَيْحَكَ إِنْ أَبَيْتَ إِلاَّ أَنْ تَصْنَعَ، فَعَلَيْكَ بِهَذَا الشَّجَرِ، كُلِّ شَىْءٍ لَيْسَ فِيهِ رُوحٌ.
Here the translation "puts life in it" is a bit problematic as the term used here is Rooh روح (that's why I added the original text inside the quote), which is usually translated as "soul", while the trees have life (hayaat حياة), and Allah said in the qur'an:
Have those who disbelieved not considered that the heavens and the earth were a joined entity, and We separated them and made from water every living thing (وجعلنا من الماء كل شئ حي)? Then will they not believe? (21:30)
This means anything that requires water is living, but the soul is inserted by Allah only:
And [mention, O Muhammad], when your Lord said to the angels, "I will create a human being out of clay from an altered black mud. (28)
And when I have proportioned him and breathed into him of My [created] soul, then fall down to him in prostration." (15:28-29)
And they ask you, [O Muhammad], about the soul. Say, "The soul is of the affair of my Lord. And mankind have not been given of knowledge except a little." (17:85)
See also 32:8-9 and 38:71-72. And therefore conquering with him by creating something similar to his creation -by drawing or shaping- is a huge sin and the "creator" will be challenged to insert a soul inside his "work" or "creation" on the day of judgement.
So trees are actually living beings as they are living, but they have no soul.
But after checking several definitions which all agree on the faultiness of my statement I have to correct it and say:
"Plants in Arabic are regarded as living beings, but they have no soul. The major -and only known to me- condition for an object or being for being prohibited to draw or imitated by any means is whether or not it has a soul"
The origin of my mistake came from the logic: everything which has a soul must be living (wrong as we know by now), but being alive and not being able to move is equal to not living, not living however means that an object/item must be regarded as inanimate and this is a (wrong) conclusion from ibn 'Abbas's statement above.
From this -hopefully now correct- perspective it is permissible to draw landscapes, like rivers, trees, mountains, stones and rocks.
Whether the drawing shows the being fully is a secondary criteria for a prohibition
However drawing a part is not regarded as equal to draw a full object/being:
The Messenger of Allah (ﷺ) said: Gabriel (ﷺ) came to me and said: I came to you last night and was prevented from entering simply because there were images at the door, for there was a decorated curtain with images on it in the house, and there was a dog in the house. So order the head of the image which is in the house to be cut off so that it resembles the form of a tree; order the curtain to be cut up and made into two cushions spread out on which people may tread; and order the dog to be turned out.
The Messenger of Allah (ﷺ) then did so. The dog belonged to al-Hasan or al-Husayn and was under their couch. So he ordered it to be turned out.
(Sunan abi Dawod and Jami' at-Tirmidhi)
This basically is a permission to draw body parts for the case of necessity for example in anatomy books etc.
Some sources in Arabic:
islamweb fatwa's #4138 (it is more god-fearing not to draw parts of humans) #32282 ( a tree has life but no soul).
And in English islamqa #23354