Right of Survivorship in a Joint Property

Hi,
My doubts is regarding Joint Property (self-acquired).

For instance, if a husband and wife own the joint property (nothing specified in conveyance deed about being tenants-in-common or joint tenants), how is the property transferred on the demise of either one of them?

As far as I read on internet, ownership will be taken as tenants-in-common and the succession will be according to the will. In case of no will, it will be succeeded by legal heirs

1. By default, is the ownership really treated as tenant-in-common? If so, is there any way to change mode of ownership from tenant-in-common to Joint Tenancy, or Tenancy in entirety?

2. In case of Tenants-in-common, does the will supercede the rights of heirs, and if one of the husband and wife mentions a third person (out of family) in the will, can the spouse and legal heirs contest the will?

Thanks in advance !