how do I transfer only part of my documents and apps

I can think of a few options to manually transfer selected files and app's from one Mac to another.

File Sharing over your local network. This work great for transfering small amounts of data.

see > Set up file sharing on Mac - Apple Support

Using Air Drop. Again. this work great for transferring small amounts of data.

AirDrop on your Mac - Apple Support

Using an external USB flash drive or hard drive. IMHO the fastest and best.

Use external storage devices with Mac - Apple Support

Manually.

Sneakernet

Copy the ones you want onto a EHD and then copy to the new Mac.