When building websites with other team members; where do you host the development area?

Let's say my companies website domain is example.com; where is the 'best' or preferred place of storing client's website builds (like the websites the company is working on-so team members can work on it remotely).

Currently, and I think its the wrong move, our customers' websites are being stored in a folder on our website. example.com/clientswebsite. I feel like that is not the best place since it may mess up SEO(and just not preferred).

Where do you guys place those at? Another domain name that stores all the builds, a subdomain of your website, or some other location?

Thanks for you help.