If you didn't have your settings set to save your TikTok videos to your device every time you posted a new video, you'll need to go through manually.
There are a few ways to do this.
The quickest way is to press and hold down on the video you want to save, whether it's your own or someone else's. An option to "save video" will appear, allowing you to add it to your camera roll or share it via email or text.
You can also hit the share button under the comments icon, which appear on the right side of videos on the app. From there, you can save the video to your camera roll or post it on other social media sites.
If you want to save videos you've liked in the past, go to your profile and click the "heart" icon under your bio. That tab shows a running list of all the videos you've ever liked.
To make sure all your videos save automatically in the future, turn on the "save to device" option before posting a TikTok. It will remember your selection the next time you post a video. You can see your saved TikToks in your smartphone's camera roll.