Symplify is based on unique users, which means if a user converts multiple times in different sessions, there will be only one unique conversion registered.
GA4 is event-based. If you want to see their value of unique visitors you need to use their "Total users" metric.
Symplify and GA4 are two different tools and these store visitor information and collect visitor data in different ways.
Symplify uses cookies to store the user ID. If the cookie is removed (by for example by Safari's ITP) the visitor will get a new user ID and thus be seen as a new visitor in Symplify.
Google also store the user ID in a cookie. They do however support Server-side tagging / server-managed cookies which, if used, will help retain the cookie.
Symplify also have a way to retain the cookie. This is done by (re-)setting the cookie from your server and thus for example bypassing Safari's ITP. This however needs careful implementation and should be tested thoroughly after implementation.
Important!
If you are comparing the number of visitors in GA4 vs Symplify for a specific Symplify project you need to set up segments in GA4 for the variation IDs of the project. If you do not do this GA4 will show you visitors that might not even been allocated the project.