I want to use mongodb on my local server. will it be free or paid and will all its services be free?
MongoDB Community is free and can be used on your system. MongoDB enterprise is the paid version of MongoDB. Community and Enterprise have basically the same database features. There are some security features that are in Enterprise only.
MongoDB Atlas which is the self managed service also has a free M0 cluster you can use. Atlas has more features than self managed (ie Atlas Search, Charts, etc)