Determine the consumption of air-to-water heat pump from the total electricity consumption

How can I now calculate the consumption of the air-water heat pump from the costs?

Without a separate meter that measures the heat pump consumption or the system's own data, not at all. Check the settings, there should be a menu item for consumption somewhere. For the future, you can simply have a standard meter additionally installed in between.

Often you can also check the consumption on the heat pump.

You can. But the numbers are not correct at all, as the devices often round to 100W. So with 351 watts consumption, maybe 400 watts are counted. And then it is also unclear what exactly is being measured. Compressor? Pump? How exactly is it measured? If you assume +/- 20% for the internal meter, it roughly fits.

It is of course hard to guess what the second electricity meter is for. It is certainly not for the heat pump. Too much heat was produced for that. Where are the heat meters installed? One on the heating and one on the hot water? Is there a separate apartment? Overall the consumption is rather high. What kind of house and heat pump do you have? Often you can also check the consumption on the heat pump.

We have an Elco Aero split 07 heat pump. Yes, as far as I can judge, the heat meters are disconnected once on the hot water and once on the heating. We have a new building in which we have been living for 1.5 years. The values (except electricity) are therefore each from 1.5 years. There is no separate apartment.

You can. But the numbers are not correct at all, since the devices often only round to 100W. So with a consumption of 351 watts, maybe 400 watts are counted. And it is also unclear what exactly is being measured. Compressor? Pump? How exactly is it measured?

If you assume +/- 20% with the internal meter, it roughly fits.

Which meter do you mean? The extra meter in the distribution box?

I have just found consumption values. However, they do not match.

Once retrospectively for the last 4 months:

Hot water 09-12/21: 36/54/53/69 kWh
Heating 09-12/21: 0/83/178/290 kWh

For one year:
Hot water: 69 kWh
Heating: 331 kWh

One would probably have to start with "Adam and Eve" here…

So we're talking about 2021 and not 2020. (House only exists for 1.5 years).

A 7 kW heat pump is also known in the meantime.
Helpful would be the mentioned heat quantities and all meaningful meter readings. Each with the correct units.

Information about the house (150 sqm…300 sqm, KfW-thing, with/without heated basement, number/age of residents, location, room temperature(s), etc.) would also be good for "estimating the consumption".
The heat pump certainly also has one or more meters integrated.

With 6800 kWh annual demand, I would guess 4000 kWh heat pump and 2800 kWh household.
But let's wait for the further numbers…