3 completely different products.
Otzar Hachochma – Over 100,000 seforim, including most new and current seforim. Basically a complete Otzar Haseforim at your fingertips. They are in their original tzuras hadaf, with a very decent search engine. Expensive, though it is on sale during Shavua HaSefer.
Bar Ilan – Only about 2000 seforim. All digital (not tzuras hadaf), but fully searchable and hyperlinked. Perfect for research and cross referencing. Primary focus on on Halacha, with most Tshuva Seforim included. Since its digital, easy to cut and paste. Catered to a different market than Otzar Hachochma. Cheapr than Otzar Hachochma, and there are occasional good deals.
HebrewBooks – Poor man’s Otzar Hachochma. Mostly older prints and the search engine is pretty weak. Free.