There are hundreds of great lat exercises my friend. Now if you're comparing exercises to lat pulldowns, then you are referring to vertical pulls yes? Well you can try pullups, chinups, one arm lat pulldowns, lat pulldowns with bands, etc. Even the classic cable/band pullover can help build up those lats. Lots of options! :)

All those vertical exercises Alex gave as suggestions are great! If you're just looking for general lat exercises, you can also do pendlay rows, dumbbell rows are actually great for the lats, and if you do a cable row with a wide, underhand grip (or a barbell row with an underhand grip) both of those will target the lats a bit more.

How many reps and sets should a beginner start out with?

@S133003836 It depends what your goal is my bro? Gain muscle according to your profile? Then the 8-12 rep range would be a good start. Then once you've been training for a few weeks/months and you know a bit more about what you're doing with performing your exercises, you can even go down to the 4-6 rep range to work on a bit of extra strength gain. Watch this video too!

How many different exercises is it okay to do for each muscle group each time you workout?

@S133003836 I would stick to 2 - 3 exercises for your bigger muscle groups (chest, back, legs etc.), and then more like 1 - 2 exercises for your smaller muscle groups (biceps, triceps etc.).

I've seen your stretch, flex, overload series. Can beginners start out with that, or is it more for the advanced lifters?

@S133003836 Beginners can use it too! That's the hardgainer sereis, so it's more if what you've been using to try and build muscle isn't working, then you could try that to get the gains going. But as a total beginner, yes you can use those workouts!

How does bench press with an underhand grip differ from regular bench press?

@S133003836 A reverse-grip bench press (underhand grip) is going to help you to target your upper chest more! Possibly even better than an incline press, but you probably won't be able to lift as much weight with a reverse-grip bench press.

What's the difference between forward lunges and backward lunges?

@S133003836 There isn't much difference really. You end up in the same position at the bottom of the movement. They'll both target the same muscles too 😊 

When doing bench press with a barbell, should you bring the bar all the way down until it touches your chest or stop just a little above it?

@S133003836 Make sure it comes all the way down and touches your chest on every repetition - full range of motion bro, always!

Are there any exercises that beginners should not start out with?

@S133003836 None at all. Beginners, as long as they don't have any injuries or ailments preventing them from doing something, can do any exercise! That's how you learn proper form, by practicing from the very first day!