If you are using Salesforce Community and you have network Id of the community but I don’t know how to get Base URL of the community. Here is a way to get the community base URL using its id in apex:
public static String getCommunityURL(String communityId){
String base = Network.getLoginUrl(communityId);
base = base.replace('/login', '');
return base;
}
Usage:
Id communityId = Network.getNetworkId();
getCommunityURL(communityId);