You are a bot. So, people interact with you (directly or in a group). When a message is sent to your bot, you would receive a callback that includes the message details (including the chat_id, the sender chat id, the message id, and all other details). Something like this (see message->message_id and message->from_id):
{
"update_id":1111,
"message":{
"message_id":111,
"from":{
"id":1111,
"is_bot":false,
"first_name":"...",
"last_name":"...",
"username":"...",
},
"chat":{
"id":1111,
"first_name":"...",
"last_name":"...",
"username":"...",
"type":"private"
},
"date": 1111,
"text":"...."
}
}
So, you can store those details and use them to forward that message to another chat. In each chat (unique chat id), each message has a chat id (which is usually an incremental number) and by these two identifier, you can uniquely select which message from which chat should be forwarded.