To make a group thank you video, send everyone the same short request (what to say, how long, which way to hold the phone and the deadline), collect the clips through a chat, email or a shared folder, then join them in one editor with a title, a closing message and a song. One organiser gathering ten- to twenty-second clips from each person is the whole method.
The maker on this site handles the last part: it works with the clips once they are on your device and saves the finished video there, without uploading anything. It does not collect messages from other people for you, so the collecting is done with whatever your group already uses. For a whole class thanking a teacher, see organising a class video, which adds consent and fairness for children.