Adult manga is the wide label, and the stores use it loosely. MangaPlaza tags 1,564 Adult Romance titles as Mature (18+), and BookWalker hides 1,937 of its 9,687 ecchi books behind a setting. Neither has a hentai shelf in the research we checked.
Hentai manga is the narrow one: sexually explicit Japanese comics. The licensed English supply comes mainly from FAKKU, which sells books, doujinshi and magazine chapters, and Irodori Comics, which sells doujinshi as DRM-free PDFs.
Doujinshi are self-published comics, either fan works based on existing series or original stories. A large share of hentai is published this way. Irodori Comics charges $5.25 to $14.95 for a single book, and FAKKU priced one 63-page doujinshi at $8.50. Buying from the Japanese circles themselves goes through Fantia, pixivFANBOX or Ci-en, which are in Japanese and have lost card options.
Mosaics come from Article 175 of Japan's Penal Code, which makes distributing obscene drawings a crime. Japanese editions cover genitals for that reason, and the English licensors say theirs are uncensored.
Phone readers should know that store builds of manga apps carry the mature-rated shelf only. Manta's iPhone app is rated 16+ and opens edited versions, while the 18+ full versions open in a browser.
The best place to begin is the list of hentai manga sites. For mature romance and BL that stays short of hentai, read the adult manga stores. Readers after the uncut editions will want the page on uncensored hentai, and phone readers the hentai manga apps list.