Hi,
I have a working server A and a new server B. i intend to restore the backup of A into B. some of the database in A has user alias in them. When i restore a database in B, will the alias be restored as well? or i have to manually create athem again? Thanks.

Yes it is restored (sysalternates)
Your suid is probably not the same on server A and B

how do i prevent the trouble of trying to figure out the suid conflict? cos my new server B is a reused database which some id already.

Another question is that there are several system tables in the database. Is is needed to migrate all over, especially the roles part? Thanks.

syssrvroles
sysloginroles
syslogins
sysroles

when we transfer the logins over, is the password transferred over as well?

In the table syslogins the password is stored (encrypted). BCP out/in also transfers the password.