ISO 9001, ISO 14001, ISO 45001, and ISO 27001 share structure: context, leadership, planning, support, operation, performance evaluation, improvement. Running them as four systems is how you get four internal audits, four management reviews, and four ways to lose a record.
An integrated management system uses one set of shared procedures and standard-specific operational controls where they belong. The registrar can often sample them together. Your people learn one way of working.
WCH implements IMS programs as a first build or as an upgrade when a second standard is added. The quality core stays intact; environmental aspects, OH&S hazards, or information-security risks are added without cloning the manual.
The decision of which consulting firm you select will have lasting effects. An IMS built as a copy-paste of four templates is worse than separate systems. An IMS built around your processes is the lowest-cost way to stay certified.