Tachiyomi

Warning

Tachiyomi was a manga reader, it has been discontinued. It’s successor is Mihon

Tachiyomi is discontinued = no more updates. It’s still working now but won’t guarantee it’ll continue working in the future, especially if you upgraded to new Android versions.
Tachiyomi spiritual successor is Mihon[1], although other alternatives also available like TachiyomiJ2K, TachiyomiSY, Yokai, etc. Though Mihon is the one that looks and behaves exactly like Tachiyomi… for now.
Currently Mihon is still compatible with Tachiyomi’s backup file, so you can just backup Tachiyomi settings and restore it in Mihon.
You’ll be asked to give a folder access to Mihon, just pick your old Tachiyomi’s folder where all your downloaded and local manga files reside.
Because of the legal problem, Tachiyomi and Mihon now has become a reader-only and won’t provide extensions anymore, you have to provide your own manga files -or- use a 3rd party extensions repo, which isn’t verified by the Mihon/Tachi developers, so watch out on what you install.
To get the extensions, you can add in repo into Mihon (More > Settings > Browse > Extension repos), most commonly used repo that I know of is Keiyoushi[2], just go to the link and follow instructions.
After installing the repo, go to extensions and install whichever you want, after installing you’ll see a shield icon with X inside on the side of the extension, tap that and pick ‘Trust this extension’. You have to do this every extension update.