Multiple database EF Core

services.AddDbContext<ApplicationDbContext>((scope, options) =>
            {
                var context = scope.GetRequiredService<IActionContextAccessor>();
                string connectionString = "DefaultConnection";
                var actionContext = context.ActionContext;
                if (actionContext != null)
                {
                    var routeValues = actionContext.RouteData.Values;
                    if (routeValues.ContainsKey("community"))
                    {
                        connectionString = routeValues["community"].ToString();
                    }
                }
                options.UseNpgsql(
                   configuration.GetConnectionString(connectionString),
                   b => b.MigrationsAssembly(typeof(ApplicationDbContext).Assembly.FullName));
            });

Looks good, but here's a slight nitpick...

connectionString = routeValues["community"].ToString();

Won't this part just get the community name / community id from the routing data?

It's not really a connection string, the variable name is a bit misleading.