Hello Simon, there are different ways to save a post on Instagram. What is your device ? IOS or Android or etc. ?
If you want to save the posts on Instagram and find them again in the archive section in Instagram ,you can use the Instagram help website as a source. Here is the link, Instagram has defined the save Icon for each post and to save the reels you can use three dots icon and then press save.
On the other hand, if you want to save the posts in your device and local storage you should use the external apps.
I suggest that the writers of this website write the full article about saving the posts on Instagram.
I hope this helpful