FLV is a container format that is usually used by Adobe Flash Player in a web browser. It can store video, audio, and text.
Clideo's FLV Merger allows joining different videos, images and adding audio. You'll also be able to download the final result in any format you need.
The tool works online, so it won't take up the memory of your device. In the free version, each file that you add can be up to 500 MB, if it turns out to be not enough for you, you can always subscribe to Clideo Pro.
How to combine FLV files into one
Add FLV files
Click on the "Choose files" button and select the necessary videos from your device.
If you want to add a video stored on Google Drive or Dropbox, call a drop-down menu by clicking on the small arrow. There will be also an option to upload a file via a URL.
Note that you can add not only FLV but files of any format.
Merge FLV and other files
Use the menu with all the options from the previous step to add more content. Rearrange them to your liking.
Set one of the available aspect ratios and crop the frame.
Upload a soundtrack, click on the scissors icon to cut it, and adjust its volume.
When you're all done, choose a format and click on "Merge".
Save joined FLV
Take a look at the result. If you like it, download back to your device or cloud storage. Otherwise, you can go back to editing without the need to start over.