You can find the ID in the URL when viewing public avatar details on the official VRChat website after adding them to your favorites. How to Download Your Own Avatars
No. You cannot just type vrchat.com/download/avatarID in a browser. You need a local application to handle the decryption. vrchat avatar id downloader link
Some community-made tools are used for "avatar recovery," though they are often flagged as risky or against TOS if used to take others' work: You can find the ID in the URL