How do you approach hiring for platform teams? Is it better to source new talent with platform experience or to promote current staff with inside knowledge of your org’s architecture?

The point that can be easily missed in platform hiring is that: platform development & usage is a team activity.  So, look for teamwork related attributes in addition to technical capabilities. Rest of the points are mentioned in earlier posts - so not repeating.

We do a blend of both, internal and external talents. Many of our key platforms are currently either revised or newly established. Here, we benefit very much from external experience and new perspectives.

Ultimately, the best approach depends on your your organization's specific needs, budget, and strategic goals. A mix of internal promotions and targeted external hiring can provide a balanced solution that leverages both internal knowledge and external expertise.

Helps build morale, but they must have the a growth mindset (so they can develop new skills).
External, brings you the skills, but culture, historical context sometimes takes time and is frustrating.

We look to balance the team with experience and internal staff.  Assuming we do not have the expertise, we have hired either an experienced leader or SME.  We need the leadership to create the platform operating model and operationalization plan.  We also look to develop growth opportunities for current staff, so will train team and assign them to the platform team.  

I think it is a mix. If you want to refactor portions of the platform you want some fresh eyes on it. I would highly recommend a very experienced new hire, combined with strong internal candidates. I would also suggest the do Mob Programming for a significant portion of the time.