how does these cheap flight search websites works

zugu.co.uk
cheapflights.co.uk
thisistravel.co.uk
lastminute.com

As far as i know these sort of site dont sell tickets whereas they direct to those websites/travel agents.

Do these sort of websites shares the database of those agents/companies who sells tickets.

They make deals or have contracts with other websites/travel agents and they get a commission for every person who comes through their site to buy tickets.

On the technology side, they use APIs (Application Programming Interfaces) delivered by travel agents.
Just like reseller hosting.