In trying to learn Puppy Linux, many of my flash sticks have been used for storing downloads and making bootables.

How to empty and/or format them, please?

Thanks.

google is your friend
https://phoenixnap.com/kb/linux-format-usb

There's no need to "empty" if you're going to use them for burning .isos (making bootables), and doing so can reduce device life expectancy. Burning an .iso file to a USB device as instructed destroys whatever content the device contained. .iso files contain the filesystems and partitioning that contain the software. There are exceptions, such as Ventoy.

If you wish to use them for storing files, they may need to be repartitioned and a suitable filesystem installed in same manner as to a HDD or SSD. Or, they may simply need to have the files on an existing filesystem deleted in similar manner as deleting files on your installed system (empty). In the latter case, you may need to use superuser permission, root login, or sudo, or su -, for deleting to be allowed.

To install another .iso, just (image) copy it to the pendrive, it will overwrite the previous one.

If you want to use them for storage, you need to give it a new MBR, partition, & filesystem - (I use fdisk).