insurer wants to know what secondary activities my company has what do I enter

Secondary activities are the SBI codes in addition to your main activity

The insurer is not asking here about individual jobs, but about the registered activities of your business in addition to the main activity. In practice, the answer is: the secondary activities listed in the Trade Register, with their SBI code. That is the same question as in which sector the undertaking operates, but one level deeper: not only the main code, but also the codes registered as secondary activity.

So enter what is literally in the extract, not what you actually do if that is not registered. For example, if you also lease business equipment in addition to your main activity in construction, and that leasing activity is registered as a secondary activity, then it counts. If you do something sporadically without it being registered, then strictly speaking it is not a secondary activity in the sense the insurer means — in that case enter what the register says, and optionally explain what actually happens in practice if the question allows for that. Insurers usually ask this because the premium risk differs per activity: a secondary activity that is not covered or not reported can become a point of discussion in case of damage. That is a reason to consult the extract, not to make your own assessment of what "actually" counts.

Where this comes from: the Trade Register

The secondary activities of a business are part of the data recorded for each business under the Trade Register Act 2007, in addition to the main activity. The extract from the Trade Register is thus the supporting document: it shows both the main activity and any secondary activities with their SBI code, and that is the source this answer comes from — not your own assessment of what "roughly" belongs to the business. If an activity is not listed in the extract, then there is no source for this answer to base that on, however real the activity is. If a secondary activity is missing that should be there, then the solution lies with the Trade Register itself: update the registration, not stretch the answer to the insurer. The same precision applies to related questions, such as the registration number under which the enterprise is registered: the answer is what is registered, not what feels correct.