Add check comments to db.ts file

src/db.ts

@@ -6,10 +6,12 @@ import { checkReplicaSet } from "./utilities/checkReplicaSet";
 let session!: mongoose.ClientSession;
 
 export const connect = async (dbName?: string): Promise<void> => {
+  // firstly we check if a connection to the database already exists.
   if (mongoose.connection.readyState !== 0) {
+    // if the connection state is not 0, it means a connection already exists so we return.
     return;
   }
-
+  // if no connection exists, we try to establish a new connection.
   try {
     await mongoose.connect(MONGO_DB_URL as string, {
       dbName,