Slipping by BTs undetected

Hold your breath while moving.

As long as you don't get too close, BTs will detect you by your breathing only.

Additionally, as long as you're in BT territory (and as long as BB is in good shape), your Odradek will always point towards the closest BT. Simply keep moving forward until the Odradek is pointing forward. When it does, use its scan feature to see where the BT is, and change course to avoid it.

If you need to breathe, just stop moving. BTs are much less likely to detect your breathing while you're staying still. Just make sure you're not too close to a BT when you do so. Your Odradek will start moving frantically if you're too close to a BT.

This will allow you to quickly get through BT territory undetected.

As you progress through the game, you'll gain the ability to stealthily kill BTs. This will allow you to get through BT territory even faster undetected.

Like before, hold your breath while moving, but keep moving even if your Odradek points forward. Pay attention to your screen for a prompt to kill the BT, and immediately press the associated button if it appears on screen. With proper timing, Sam will kill the BT even before it notices him. Even if you're slightly late, Sam will still kill the BT before it manages to grab him. This won't alert the other BTs. Feel free to take a break and breathe after killing a BT.

Just drive

When you gain access to vehicles, you'll be able to completely ignore BTs as long as you have enough battery.

Simply drive through BT territory without worrying about them. The BT will most likely catch you, and when they do, your vehicle will be stuck in the tar. When this happens, immediately use your Boost and don't let go. Your vehicle will eventually manage to get out of the tar before the BT can get ahold of Sam.

Keep in mind that boosting drains your battery faster than driving, so if you're low on battery, build a generator or otherwise find a way to recharge before crossing BT territory.

Also keep in mind that while this strategy works very well, it is not 100% reliable. Very rarely, the BT may succeed in dragging Sam out of his vehicle before he gets unstuck. The vehicle will then sink in the tar and become unavailable until you either defeat the boss, or escape from the BT territory. Cargo contained within the vehicle will be spread on the ground around the vehicle, so don't forget to pick it all up before leaving.

You can use you scanner called Odradek.

When you are in BTs territory it will automatically point to the closest BT around. It will also start clapping more the closer you get to any BT.

When you activate actual scan, it will show BTs for short period of time.

However, Odradek will stop working if your BB is too stressed. Also, there will be some story related events when you can't use it.

Also, you are supposed to move slowly in BTs zone, otherwise they will detect you right away.