How to do search in eloquent for inner tables in laravel

Use whereHas (or in this case orWhereHas) to constrain the query using the relationships:

$discover = Discover::with(['category','subcategory'])
    ->where('status','1')
    ->where(function ($builder) use ($searchString) {
        $builder->where('name','like',"%{$searchString}%")
            ->orWhere('description','like',"%{$searchString}%")
            ->orWhereHas('category', fn ($builder) => $builder->where('name','like',"%{$searchString}%"))
            ->orWhereHas('subcategory', fn ($builder) => $builder->where('name','like',"%{$searchString}%"));
    })
    ->orderBy('created_at', 'DESC')
    ->get()
    ->toArray();

Note I assumed both categories and subcategories table each have a name column that you want to filter on - change as appropriate. Also logical grouping of the OR constraints is necessary!