Object placement for level design

I'm currently working on an isometric game with WASD movement and it's going to be 3D low poly.
I've only worked on small projects from courses before so I never built a full project.

I'm going to be drawing the assets myself so my question is how I should approach designing the levels in unity?
a few basic features: it will be mostly an open field with trees, stones, boulders, rocky tiles for roads, etc...
I will probably not use slopes and mostly be all on the same level, but that is subject to change.

I'm not sure what tools to use to draw the level itself:

• should i use terrain for tree painting
• should i use polybrush since most assets will be random objects
• should i manually place stuff
• is there a way to combine the tools

I've tried searching and there are a few guides to explain what each tool does, but i couldn't find tutorials towards building larger fields and scenes.

TLDR: what tools should i use to build my scene?