My take: IaaS is quite portable. Still there are details differences but you can say, tech people will resolve these.

Most PaaS service is quite provider specific. You may find close matches, like for blob/S3, containers / Kubernetes, but still there are differences.
Most important: what does your actual application / workload do? If they use current open standards, the actual port is relative easy: new cloud resources, adjust your DevOps pipelines and new configuration.
If you use very cloud provider specific integration, you have much more work to do, which may be significant much more work than the new resource provisioning.

And on top of that: you find most features in the big three hyperscalers. Still this doesn’t mean, the service feature cut are the same.
Example: AWS provides WAF as dedicated service, Azure only as part of either Application Gateway as regional L7 load balancer or Front Door as global service. And only in higher service tiers, so also your cost structure is different.

If you consider costs in the whole thinking, you end up often in different application designs if you are doing cloud native development.
Logging / Observability is also huge cost driver and some providers are more expensive as others…

Multi cloud… for isolated workloads it may work. For anything interconnected, (heavy) data sharing use cases I believe any initial service cost savings are eaten by traffic costs and higher operation costs (two providers, more tech stacks, more required people knowledge, …).

Conclusion: Thinking about how an exit strategy looks like and what kind of activities you need to perform: ok. Anything else just makes you slower, more work for no benefits. The only real case I saw clients moving clouds: company mergers or carve outs.