On Instagram, you can save your favorite Reels to watch them later.
Save Reels on Instagram
When you see a Reel you like, tap the three dots button on the upper right corner and select Save from the shown menu.
See your saved Reels on Instagram
Now, to access your saved Reels, go to your profile and tap the three lines button from the upper right corner of the page.
From the menu tap Saved. Open the saved posts collection and go to the Reels middle tab.
To later remove Reels or posts, use the Select and Unsave buttons.