Radicals do not have anything to do with meaning.
I'll start off with a rather lengthy correction to a common misconception. Radicals (部首, literally section 部 header 首) are just a dictionary organisation tool, specifically used for texts which collate and refer to a large collection of characters, serving as the head of each section of a dictionary. They are ...
According to 《通用规范汉字表》("General Standard Chinese Character Table"), "左部件或左上部件末笔为横的，应该变形为提"(The end stroke of the left part or the upper left part is horizontal character stroke(横), it should be changed upwards character stroke(提)). "骑" meets this condition. "马" is its lest part, the last stroke is horizontal character ...
Is 纟 obtained by simplification of 糸 ？
Yes, 「纟」 is a cursive calligraphy abbreviation of 「糸」 with its strokes straightened later, and in print form, it exclusively appears in Simplified Chinese as a component, but only under certain conditions. If you want to write 「糸」 by itself, it is still 「糸」 and not 「纟」.
Like other conditions on these kinds of ...
The standalone character 糸 pronounced si1 is a variant of 丝 (丝的异体字)
It is written 糹(traditional) and then 纟(simplified) when it appears as a radical on the left side of a character. According to the 简化字总表:
[...] 不论在一个字的任何部位，都可以使用，其中 “讠、饣、纟、钅”一般只能用于左偏旁。这些简化偏旁一般都不能单独使用。
When it appears in other positions, it keeps its full form 糸:
纟 derived from 糸, which is the silk. (looks very like)
Like many other simplified Chinese Characters, 纟comes from 草书, such as
訁 -> 讠
飠 -> 饣
糹 -> 纟
釒 -> 钅
most of the characters with 纟are related to textiles ( 纺织品 ) , such as
refer to: https://baike.baidu.com/item/%E7%BA%9F/8457525?fr=aladdin