A user with access to a SECURED field in a record can CHOOSE to share it with another user or team. The user can only give the access that they have on the record. For example, to share the record and grant Update PRIVILEGES, the user must have update privileges.
You can share a secured field on a particular record with READ and/or Update with a security principal (user or team). The user or team members with whom the record was shared now have that type of secured field access only on the shared secured fields on only that particular record, even if the user or team member to whom it was shared does not have a field security profile that gives them access.

A user with access to a secured field in a record can choose to share it with another user or team. The user can only give the access that they have on the record. For example, to share the record and grant Update privileges, the user must have update privileges.
You can share a secured field on a particular record with Read and/or Update with a security principal (user or team). The user or team members with whom the record was shared now have that type of secured field access only on the shared secured fields on only that particular record, even if the user or team member to whom it was shared does not have a field security profile that gives them access.