Challenges are set by anyone. Be it a local community member, a global NGO, or a company. By participating in their challenge, you're helping provide data towards their account which hopefully they can use to help create systemic change.
You can join a challenge in two ways, either by typing in the join code you were given, or browsing the Challenges Tab in the Litterati App for local or featured public challenges to join. See screenshots below.
You can join as many challenges as you like, your data will go to all of them as long as you're within the challenge dates and radius.