There's a book called "What Mothers Do, Even When It Looks Like Nothing" which explains it really well.
With a first baby, in particular, it's a bit like walking up in just your clothes in a foreign country where you don't speak the language or understand the customs.
Your baby is born (and you are probably at least a bit injured and exhausted from that) and is utterly dependent on you for everything. You are printed by your hormones to look after your baby. For many people what this means in practice is that you are hyper-alert to the baby. You will hear the tiniest noise the baby makes even if you are in a noisy room. You will panic if apart from the baby for too long. When the baby cries, it triggers a sense of panic strong enough to be physically painful.
You don't know how to communicate with the baby at first. You have to learn what the baby likes and doesn't like, how they express pleasure and discomfort and hunger and tiredness, what soothes them and what upsets them. You get it wrong, a lot, and they cry, and that makes you panic. The panic fades a bit but doesn't go away if you hold the baby. Learning your baby takes up most of your thinking capacity, and it's pretty tiring.
If you breastfeed, you might be lucky and find it easy, but most people have trouble at first, and that involves pain, stress, and worry.
The baby feeds all the bloody time, especially in your sacred adult time between around 5-9pm. When you are learning to breastfeed you can't move out eat during this time, and your hands are full of baby. You might well be in pain.
As you start to understand your baby, you gain more freedom and time, but you are not in charge of how you spend your time. You are constantly interruptible, and every thing you do, you do on the understanding that you will stop at once if the baby needs you.
Most babies start off wanting to be held constantly, day and night, which limits time to do stuff without a baby in your arms.
Many babies cry a lot, even when you you all you can to look after them.
You will sleep in short interrupted bursts. It won't be enough.
People (or books or websites) will make you think that you are doing great it wrong. Normally, you wouldn't give a shit, but you are tired and hormonal and way more vulnerable to criticism than usual.
So basically, the time is spent feeding and holding and soothing and changing nappies and being trapped under a baby who is asleep because if you love s/he will start crying. And it's spent recovering from childbirth. But it's also spent learning a huge amount in a short time, and dealing with an enormous change in your life and not being able to start any activity with the knowledge that you will have time to finish it.