This is getting really OT. You just have to read the rules for each lounge, with attention to the access eligibility type because some groups may only be allowed access on departure. Typically, paid memberships are allowed access on arrival. DL Sky Clubs apparently allow AMEX Plats access on arrival as well. Some alliance rules only require lounge access for certain eligible groups on departure, so clubs that don't wish to go above and beyond the alliance requirements (e.g., UC) will not allow access for those groups on arrival. Some airlines also offer access on arrival as a benefit to those flying own metal (e.g., Polaris) or to own-FFP members under certain conditions (e.g., UC with same-day international flight).

Centurion Lounges used to allow access on arrival, but that changed back in 2019 (when they also restricted access to three hours prior to departure).