You're right. The Replenishment Horizon is company-level (Inventory > Advanced Scheduling, or the Replenishment report settings depending on your version). The reason it's global is that it's really a reporting window. how far ahead the replenishment dashboard surfaces manual reorder needs, not a per-site planning parameter, so Odoo keeps it simple at the company level.

The native lever that is effectively per-warehouse is Visibility Days on the reordering rules. Reordering rules are defined per product × warehouse, so you can set different Visibility Days on the rules in each warehouse. Each location gets its own lookahead window for known future demand. Pair that with per-warehouse resupply routes and per-supplier/product lead times (which already differ by where each warehouse sources from), and you can get noticeably different planning behavior per site without touching the global horizon. Practical recipe: set the company horizon to your longest-planning warehouse, then tune each warehouse down via Visibility Days on its rules.

The honest limitation: native replenishment is reactive : min/max + lead time + a visibility window. It doesn't forecast demand differently per warehouse (different velocity, seasonality, trends per site). If that's what you're really after, each warehouse planned on its own demand pattern. that's beyond stock Odoo and needs a forecasting layer that models each location separately.

Full disclosure, I build one of those for Odoo, so I'm biased  but the Visibility Days approach above is the right native answer to try first. Happy to go deeper on the per-warehouse forecasting side if you head that way.
'niyu_smart_stock' is already published on Odoo App Store and is free to download.

I’m interested but

I have the online version, as far as I know no apps can be used in this version 

: )