How to write literature review when several systematic reviews already exist
Your systematic review (SR) need to fill a gap in knowledge that was not covered in the existing SRs.

By filling this gap, you will be building on, extending, and/or updating the existing SRs and the related literature.

Normally, the research question in your SR should therefore be novel and not duplicate of those in the existing SRs. This will produce different and novel findings.

In the discussion/synthesis of your SR, demonstrate how your findings fill the gap, and put your SR in the wider context: your narrow (and possibly wider) field, the exiting SRs, and other related studies.

Also see this question: A systematic review already exists in the literature. When is a newer one required?