Some of my customers just give me the cloud credentials -- they have no idea what any of that means, it got dropped on them when the last sys admin quit or stopped returning emails. Some know how to manage their cloud assets and create an IAM user with appropriate permissions. A few have made me work with/through their IT department.
Generally I ask for a privileged user login to their cloud control panel (AWS, GCP, Azure, Rackspace, Digital Ocean) and an ssh login with sudo permissions. I may get some or all of that, depends on what they need. My involvement ranges from reviewing existing cloud setups and offering advice to fully managing cloud infrastructure and servers.
I kind of fell into this from programming work. I would notice problems in the customer's infrastructure, or they would tell me about problems, and I told them I could help so they gave me the keys. I tend to keep my customers for a long time (years, a couple over a decade) so they trust me already.
You might look at consultancies that focus on security and see if you can work through them. Security experts usually need the same kind of access I need. Very often the first time a company needs a cloud expert/system admin is when they get hacked or something crashes, so security consultancies and IT support companies get those calls, and you can freelance for or through them.
A cloud expert/engineer certification can't hurt. I don't have any certifications but I have gone through the AWS materials. I got started with cloud admin early-on so I've kind of grown with it as it exploded. I have prior system admin experience that helps.
Lots of smaller companies have cloud infra that they paid someone to set up for them, but that person/company has moved on and doesn't maintain or support the customer anymore, so that's where you look for opportunities.