Best way to make multiple tiktok clips out of a lengthy video for free?

Opus Clip has a free tier. The paid version is better but it will get the job done for you.

Just be aware if it's not your video then you have no right to monetise the TikTok account and using other peoples video's can get you banned if the copyright owner complains.

If it is your video you could try something like:-

Video Candy online:
https://videocandy.com/video-splitter.html

Video Splitter on Android:
https://play.google.com/store/apps/details?id=com.psma.videosplitter&hl=en_GB

Or iPhone:
https://apps.apple.com/us/app/video-splitter-no-watermark/id1299543737