https://alternativeto.net/software/ticktick/?license=opensource

Super Productivity has some integration limitations, but generally been working great for me.

tasks.org is pretty excellent

jtx Board

Significantly depends on your needs. I need a full functional web client, what makes list of free alternatives basically empty.

Try Vikunja. Project Segfault has an instance, but you can host your own if you want.

I read that title as “Is there a FOSS alternative to apps like TikTok?”