Although clients can send invitations to jobs, it's rarely the main way of finding work on Upwork (at least in my experience). I have only had previous clients send me invitations after I completed work with them in the past and they wanted to work with me in the future.
When clients post they also get a selection of freelancers that they can choose to send invites too... I'm not really sure how the algorithm selects those freelancers, but sometimes they are not very well suited matches.
Long story short, Upwork is mainly a platform where people post jobs and clients apply, not the other way around like some other freelance platforms.
