Turn on Bulk edits, select the campaigns and click Duplicate in the bottom bar. In the Duplicate Campaign(s) modal, choose what to include: Campaign + Ad Groups + Ads (full copy), Campaign + Ad Groups (no ads) or Campaign Only (empty structure). Choose whether the copy starts Paused (default, recommended) or Active.

Copy name: if you leave it blank, the copy gets the original name plus "#2" (then #3, #4...), the same way Google does it. If you type a name that already exists in the account, Ads Editor warns you and suggests an available one, without creating anything.

Performance Max: duplicating works, and the copy includes the asset group (headlines, descriptions, images, logo and search themes).

To copy to another account, turn on the Duplicate to another ad account switch and select the destination accounts from the list (you can search by name and use Select all). When duplicating across accounts, you can also override the final URL, headlines and descriptions of the copied ads. Search, Display, Performance Max and Demand Gen campaigns are supported across accounts; Video campaigns aren't, due to a Google API limitation.