How to create a cell of IDs of related products (in the row of an original product)?

Try into C3:

Formula:

=IF(A3<>"",SUBSTITUTE(TEXTJOIN("",1,IF(IFERROR(FIND(LEFT(A3,7),$A$3:$A$18),0)<>0,$A$3:$A$18&", ","")),A3&", ",""),"")

accept with Ctrl+Shift+Enter (as it is array formula).
Then drag it down as long as needed.

	A	B	C
1	ID	name	related_products
2		PALACE
3	801_005	Climbing Frame for Children - Jungle Palace	801_005 - 850_176, 801_005 - 850_177, 801_005 - 850_230,
4	801_005 - 850_176	Climbing Frame with Swing - Palace 1-Swing	801_005, 801_005 - 850_177, 801_005 - 850_230,
5	801_005 - 850_177	Playhouse with Slide and Swing - Palace 2-Swing	801_005, 801_005 - 850_176, 801_005 - 850_230,
6	801_005 - 850_230	Playtower with Climbing Wall - Palace Climb	801_005, 801_005 - 850_176, 801_005 - 850_177,
7