What you need is the increase() function, that will calculate the difference between the counter values at the start and at the end of the specified time interval. It also correctly handles counter resets during that time period (if any).

increase(http_requests_total[24h])

If you have multiple counters http_requests_total (e.g. from multiple instances) and you need to get the cumulative count of requests, use the sum() operator:

sum(increase(http_requests_total[24h]))

See also my answer to that part of the question about using Grafana's time range selection in queries.

You need to use increase() function for calculating bytes usage over the last 24 hours:

increase(bytes_total_incoming[24h])

This query is executed independently per each point on the graph and per each matching time series. So every point on the graph shows bytes usage for the last 24 hours ending at that point. If you need showing per-day bytes usage on the graph, then the following options exist:

• To set min interval in Grafana graph panel settings to 24h. See these docs. In this case Grafana would query only a single point per day per each matching time series. Every point will show bytes usage over the previous day ending at the point.
• To use subquery feature in Prometheus in order to limit increase() calculations to per-day boundaries:

last_over_time(
  increase(bytes_total_incoming[1d])[2d:1d]
)

This query returns a graph with per-day steps showing bytes usage over the previois day.

If you need grouping the results by a paricular label such as car_name or type, then just wrap the query into sum(...) by (car_name) or sum(...) by (type). For example, the following query returns per-day bytes usage grouped by type:

sum(
  increase(bytes_total_incoming[24h])
) by (type)

Tye sum() is an aggregate function, which sums series values individually per each point and groups the reaulting sums by label values specified in the optional by (...) modifier.

P.s. the above queries return per-day bytes usage shifted by one day in the future. For example, they return bytes usage for August 10 at August 11. This shift can be removed with negative offset modifier:

sum(
  increase(bytes_total_incoming[24h] offset -24h)
)

P.p.s. increase() function in Prometheus may return fractional results when applied to integer counters. This is because of extrapolation - see this issue. If you want obtaining the exact increase over integer counter, then try the project I work on - VictoriaMetrics. It implements increase() function in the expected way without using extrapolation. See this article for details.

Prometheus doesn't provide the ability to query counter increase for the current day (e.g. since 00:00 of the current day). Prometheus provides the ability to query counter increase over the fixed lookbehind window (aka sliding window) though. For example, the following query returns the counter increase over the last 24 hours:

increase(ifen02[24h])

If you need counter increase since the beginning of the current day, then the following MetricsQL query can be used:

running_sum(
  increase(ifen02)
  if (
    time()+timezone_offset("Europe/Kiev")
    > (now() - now() % (24*3600))
  )
)

It is expected that this query is used for building a graph on a time range covering the current day.

This query works in the following way:

1. It calculates the increase() for ifen02 between adjacent points on the graph. VictoriaMetrics automatically converts increase(ifen02) into the equivalent of increase(ifen02[$__interval])). See increase docs and implicit query conversion docs.

2. Then it calculates time()+timezone_offset("Europe/Kiev") > (now() - now() % (24*3600)) . This query returns unix timestamps in Europe/Kiev time zone for the current day. It drops all the timestamp for the previous days. See docs for the used functions: now(), timezone_offset() and time().

3. Then the if operator leaves the calculated increase() values only for the current day starting at 00:00 Europe/Kiev timezone, while dropping all the values for previous days. See docs about if operator here.

4. Then the remaining increase() values are summed over the current day with running_sum function. Sometimes it is possible to use range_sum instead of running_sum.

Your question isn't 100% clear, but I think you are looking for sum(increase(purchases_total{deviceType='ios'}[1d])) and then use a 1d step to the query_range API with start/end covering 7 days.