botchat is something that happens mostly on whatsapp on whatsapp, it involves a bot adding random people to a whatsapp group and sending unpleasant imagery with malicious intent
open your whatsapp app, three dots> settings> privacy> groups> set to contacts only
just leave the group; dont waste time reporting it, someone else will do that for you
if you do decide to report it, dont send any messages. this is because when reporting a whatsapp group whatsapp only forwards the 5 most recent mesages to moderation