First off most of the guides you find online or even from nurseries show the wrong way to transplant shrubs and trees.

Filling the hole with new dirt is pretty much the worst thing you can do. Water and roots follow the path of least resistance.

When you have soil with higher porosity surrounded by soil with lower porosity, water collects the higher porosity soil. It has more room for water and collects it from the surrounding area. This often leads to drowning the plant or root rots forming.

The roots also are not going to push through the harder more compact soil when nice soft porous soil is available. So they circle around the hole you dug. You end up with rootbound plants long term.

You functionally create a pot in the dirt when you add in the "better" soil that you purchased.

The best way to transplant into the ground.

Bare root or minimal soil is recommended for the plant. You really don't want to be adding too much around it.

Refill the hole with the dirt that came out of it. Pack the dirt as you fill it to match the surrounding soils compaction level. Don't be shy, make it good and firm.

If you want to fertilize at transplanting add a very small amount around the top of the hole when filling it up, not at the bottom of the hole. You want the nutrients to leach down to the roots with water. If you want to add in more fertilizer/compost. Dig some small shallow holes 5-10cm from the outside edge of you hole and add in fertilizer. This will reward roots that break through the edge of the hole encouraging them to grow further.