There are three gain stages in a 741. First stage is a differential. Second stage is high gain as a darlington. Third stage is a class AB power amplifier. There are a few other features such as current limiting and a miller capacitor that helps to reduce oscillation. There's a 39 kΩ resistor that sets the bias for the entire opamp. It's pretty much genius and yet looks so simple. Multiple current mirrors to keep everyhing just right... it's beautiful.
the SV filter is a biquad consisting of a summing amplifier and two integrators it is has second order response. The integration of a HP creates a BP and the integration of the BP creates a LP. Each stage is out of phase with one another... it again is simple to conceptualize but work of genius. It gives you the ability to have 3 different filter output to TAP while giving you independent control over Q and ω. You can also change Av but it will affect Q. If you add another summing stage you can sum the LP and HP and get a great notch filter also. This is a great filter for audio use with it's versatile controls and different filters. It is also used in analog synthesizers due to it's versatility and the high Q you can achieve.