1) I gained insight __ working practices of such companies
2) I gained insight __ how a software project is done from scratch
1) I gained insight __ working practices of such companies
2) I gained insight __ how a software project is done from scratch
"Into" would be used in both of those examples.
I'm not certain of the grammatical rules that govern it but the use of on rather than into would indicate a less detailed view. For example "I gained insight on the working practices of such companies" would be more taken to mean that you knew what the working practices were, whereas "I gained insight into the working practices of such companies" would indicate that you'd had a more detailed understanding, learning more both about what the practices were and also -why- they were in place