ConversationId for channel messages are combination of channelId and messageId both can be found in payload you get webhook notification. You can reply to existing conversation by using following by building converstionId like this:
conversationId = $"{channelId};messageid={messageId}"
For reply to work, your Bot needs to installed in the team and should have serviceURL saved at some place to refer back. Here is sample code which show how you can reply to existing message.
var serviceURL = "YOUR Service URL- You get this in each bot payload";
MicrosoftAppCredentials.TrustServiceUrl(serviceURL, DateTime.MaxValue);
using var connector = new ConnectorClient(new Uri(serviceURL, MicrosoftAppId, MicrosoftAppPassword);
var conversationId = $"{channelId};messageid={messageId}";
var replyActivity = MessageFactory.Text($"This is simple reply to existing conversation.");
replyActivity.Conversation = new ConversationAccount(id: conversationId);
var response = await connector.Conversations.SendToConversationAsync(conversationId, replyActivity);
For 1:1 reply- please take a look at Sending Proactive Message documentation.
