How do I put all my photos on a USB

One option to free up space would be to move your Photos Library to an external drive. Then quit/reopen Photos with the option/alt key held down and make the external drive file the system Library. Once you are sure all your photos are there, reopen Photos with the option/alt key held down and create a new system Library on the internal. You can now delete photos from your devices while maintaining a master copy on the USB. Added photos can be exported from the new Library and imported into the Master Library. Turn off iCloud syncing when reverting to the Master Library.

Article if for iPhoto, but the basics are the same.

iPhoto: Move your iPhoto library to a new location

Photos library - Move your Library to an external storage device

Use the steps in the article Export photos, videos, and slideshows from Photos on Mac - Apple Support

And export photos to downloads .

Insert a usb stick in your Mac .Open finder , click on download select all photos ( use command and A keys on the keyboard ) and copy them ( command and C ) , take cursor on the pen drive and paste them ( command and V ) .

Once all the photos are copied , right click on the pen drive and click on eject pen drive .

Again open photo application , select the desired photos ( not all of them as you want to reduce the storage space ) and delete them .

The photos will move to recently deleted folder , you can select them and delete permanently .

Hence , the photos are transferred to usb stick and the storage space is also reduced .