I'm using the MPU9250, it measures acceleration, magnetometer, gyro, etc. Complementary filter tutorial for this is here, however this tutorial does not have a magnetometer and I do, thus I thought I could eliminate the gyro drift. Seen here:
I've found this post already in which other users state that a magnetometer is required. However, they do not go into much detail (and because it is such an old post I didn't want to bump it).
My other complementary filters are calculated as such:
float angle_x = alpha*gyro_angle_x + (1.0 - alpha)*accel_angle_x;
float angle_y = alpha * gyro_angle_y + (1.0 - alpha) * accel_angle_y;
angle z = yaw
I have my local inclination angle which I have been previously told I need and I also have my magnetometer value.
My question is; How do I combine all my values into an equation for the complementary filter?

You can use the complementary filter to fuse the yaw estimate from the magnetometer and the one coming from gyroscope integration.
– UserK Oct 17 '20 at 23:59