Experiences prefab house construction length and sound insulation

I am building with my sister. The property already belongs to us. The apartments are to be rented out.

Is the sister also a layperson in the field of apartment building construction? By the way, my question was aimed at whether you are acting here as the builder, or e.g. as the spokesperson of a building association of half a dozen families. So you have presumably inherited a building plot together and now want to develop it; after the refinement with a multi-family house, it should belong to you jointly as a whole; and presumably serve to fund your retirement.

I am already getting a written offer

I did not doubt that. However, a construction company does not advise adequately on such a project: they only show you how to arrange the building material into a structure. They do not care about the fate of your assets. Let’s assume you get divorced / your sister goes into a nursing home / your heirs consider their heirs idiots, etc., etc.: there is nothing more disadvantageous than a joint six-family house. The knowledge that Meierbau builds the building cheaper than Müllerbau will do you no good. No construction company will advise you to found a property development GbR; nor to make it into condominiums and e.g. distribute them directly to the children – assuming you have three children together (1, 2A, and 2B), then one apartment goes to child 1, one each to child 2A and child 2B; later the inheritance includes only shares in the "Two Sisters & Co Real Estate KG" (which owns apartments 4, 5, and 6 and manages all six apartments). Neither Meierbau nor Müllerbau will explain this to you – they just give you prices for 598 sqm of Poroton or 603 sqm of Ytong.

If with your budget for a six-family house you do not primarily want to "fall victim to robbers," then first get much more extensive advice here than just asking for a builder and secondly take the preliminary designs developed in the discussion to an independent architect.

together

Yes, she is also a layperson like me. We would both be the builders. The property belongs to each of us 50%.

cheaper

For our assets, we have a lawyer specialized in construction law and a consultant for financing. We will have a subdivision carried out. The loan will also be divided so that we are completely independent from each other.
The civil engineer we have already spoken with would accompany us throughout the entire construction, from the planning to the completion.
Unfortunately, we have zero experience and no contacts. So we don’t know which architect/construction company is reputable or competent.

The lawyer specializing in construction law is probably not the right expert for the risks I mentioned. A shared loan would require a divisible security. The question of who builds the house on the meadow cheaply and largely free of defects is "only half the battle".

For our assets, we have a lawyer specializing in construction law and a financing advisor. [...]
Unfortunately, we have zero experience and no contacts. So I don’t know which architect/construction company is reputable or competent.

If the mentioned gentlemen were well connected, the problem of finding a construction company would already be solved.