Officially, according to university guidelines of plagiarism, if this is handed in for credit, using a solution written by someone else, yes it is plagiarism. Even if you understand the reasoning behind it or learn how to replicate it.

However, once a TA told me while I was demonstrating my assignment solution that I could have just copied and pasted pre made code from some online resource instead of braving it myself, or admitting that I “needed to learn more how to work on this part” as online there are all optimal solutions and this was rather easy to find on multiple sources and free to use. But in the assignment it was clearly stated that solution had to be my original work.

Apparently students do this all the time in CS and also collaborate on individual assignments and argue that collaboration is important, sure is, but grades are mostly individual achievement. However, it still can be considered plagiarism by the university. Some professors don’t like it and try to give unique assignments that cannot be found online and they never give out official solutions so that you cannot share it with the next cohort of students, and others don’t care.