Good for you.
Good program to start on.

Yes correct posture and technique before putting on weights on the bar. But you can put micro plates (2.5kg?) or 5 kg just to have some feel. Sometimes just bar alone hard to have correct posture especially when you are new, because it's a bit too light.

Just make sure, back arch at all times, at almost all kind of weight lifting.

For example, bench press, there will be a space between your lower back and the bench, hence the arch.

The squats, keeping it straight, but once u do heavier, there will be a slight arch too.

Actually, make sure you brace your core. Then the slight arch will automatically be there.

Like breathe in, hold your breathe, compress your inner muscles / core, and do your rep.
Breathe out after rep ends. and repeat.

Eh forget 1.25 plates. Just go for 2.5kg.

1.25 a bit too little. 2.5kg really isn't that heavy.

oh bench press i would recommend your feet flat on the ground. Not just the front of your feet. You should be pressing through the heels on the ground.

Hmm I started on SL too, but if I'm not wrong, 2.5kg increment on each side. Not total.
That's what I did anyway.

After a while, you'll feel 2.5kg (or 1.25kg in your case) is boring.
You'll forget the micro plates and hit 5 kg increments. (for me anyway).

Hey bro.

My 2 cents.

Squats - what I see is, you are bending knees first during your descend. Try to learn the "hinge" at the hip side. Unhinge (ass / butt goes to the back) your hips, and then knees down after that. On squats, it's not the knees first that make you go downwards, it's your hips, lower back, and then knees. Like how you take a shit on the toilet bowl. Don't know how to explain, haha.

Bench Press - the bar path is never straight. Don't worry about it. They say it's almost like a J.

Bend over row - this looks like you are working out only the upper back / shoulders. Try to bend up a bit more and then do your rows.

Cheers.

Remember "Ass back first" then go down.

A few references:

https://www.nerdfitness.com/blog/strength-t...squat-properly/
http://thechalkboardmag.com/the-perfect-sq...in-2-steps#sl=4

No problem, after I looked at your video again, if you noticed, your knees are also too far front (but some people just can't help it due to their bone structure) - if you do "ass back first" it won't be so front.
Learn to balance your weight on your back heel.

Similarly drive up with the weights on the back of your foot.

Squats - better. Your legs are quite long, so you may need some time to get used to sit back. I'm quite tall as well, so took me a while too once my legs are strengthened. But yea, practise it at home against a wall for example, or a chair.

OHP - looking good.

Deadlift - I think the steps should be lift up with your body first, using your hips to drive the weight to your foot heel, (ass back again so that your knees are not in the way of the bar going up), and then only your legs straighten.

Getting better

Haha nice.

Eat more :-) fibre, protein, complex carbs.

For your chest, brace your core too then you might feel it,