The mapped logins are removed when database is restored Whenever I restore a database all my logins associated with that database are removed. How can I avoid this ?
Logins reside in master database and users reside in non system databases. Logins does not come with user database backup.  There is an exception for contained database, in which case there is no login mapped to user. Everything you need is contained in the user database as mentioned by @Nic and @Sepupic in comment section.

Read this for more details:

Basic SQL Server Security concepts – logins, users, and principals by Laurentiu Cristofor

Follow the process described here to copy logins to destination server.

How to transfer logins and passwords between instances of SQL Server

You can also use PowerShell function to copy logins.

COPY-DBALOGIN