Installing and using a GTA 5 car save mod is relatively straightforward:
A "car save" mod for Grand Theft Auto V (single-player) lets players store, recall, and manage custom vehicles outside the game’s default behavior (so vehicles persist between sessions, are spawned on demand, or kept in personal garages). Typical features: save vehicle to a slot, rename, export/import vehicle data, spawn saved vehicles anywhere, and protect saved vehicles from despawning. gta 5 car save mod
Buy the Auto Shop property. It allows you to store customer cars temporarily. While not a "mod," it lets you save and drive cars you don't own (though you can't keep them forever). Installing and using a GTA 5 car save