Is a contracting company between several companies considered an 'inan' company, and do its provisions apply to it? And how can one of the partners withdraw from it?

An 'inan' partnership is when two or more individuals combine their capital and also participate in the work. It is permissible by consensus, as it is based on agency (wakalah) and trustworthiness (amanah). A condition for its validity is that each partner authorizes the other to act on their behalf. Equality in profit or capital is not a requirement; rather, it is left to the mutual consent and agreement between the partners.