The LG V20 use to have a crop function, but it was reportedly removed with the Oreo update (I haven't taken the update, so can't verify it on my phone), so my guess is this is something LG has decided to remove and you'll need to use another app.
As B. Diddy said, Google Photos can do this, but is somewhat limited in aspect ratio presets. If you want a fair bit of presets for use on social media, Photoshop Express has a ton. I.e. You can crop to the exact aspect ratio for a Facebook cover photo, Twitter post, YouTube thumbnails, etc, or create a custom fixed ratio.