Assistance work doesn't matter too much. It's main use to iron out weaker points.

For example if youre bench pressing and feel you're weakest at getting the bar off your chest, then using dumbels or the chest press machine might help you to generate more force in the bottom portion of the lift because the weight starts from the bottom.

By the same logic if youre weakest at starting the deadlift, pressing/quarter repping on the leg press machine will help to mimic that movement.

If you're weakest at coming out the squat, then leg pressing from the bottoms-up position may help too.

Assistance work also helps to generate more volume. If you do bodyweight chin ups till failure, you can move on to the assisted chin up machine or lat pulldown afterwards, which should carry over in to improving your main movement.

Sometimes using a machine is just more practical if you're reaching failure but feel you need more volume, especially if you train without a spotter. If you fail a rep on the chest press and nothing bad really happens because it'll fall in a fix path where as fail at bench press and you'll get pinned under the bar.

Most of your effort should be spent on the main movements, the assistance exercises are just there to add more volume and target assisting muscles.

For example you wouldn't want to do heavy cable flies to improve your bench press. Do heavy bench pressing and then afterwards you can do some lighter pump work on the cables to get more volume and feel the contractions. Assistance work is usually more fun too.