You can find old chats in Teams by using the search box at the top of the screen which acts as the global search bar for the app. You can type a keyword, name, or file to find a previous message or conversation. If the chat is hidden, you can select the person's name and choose to show the chat history. Alternatively, you can use a command to ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.