Signing up currently means signing in with Google — Mem's help centre says a Google account is the only supported login, though its privacy policy also mentions Apple, so you may find more options on the sign-in screen than the help pages describe. If you do not want Mem attached to a Google identity, this is your stopping point, and Obsidian is the obvious alternative in the same category.
The free plan is real but small, and it is metered by activity rather than by time: a small monthly number of messages to Mem and new notes, task tracking across those notes, and a few hours of Voice Mode. Free is enough to find out whether the recall actually works on your own material, which is the only question worth answering first. You would move up when the counters bite — to Mem Plus, which roughly doubles the allowances, or the pricier Mem Pro, which is where connecting Gmail, Slack or Todoist and setting up scheduled routines first become available at all.
It runs on Mac, Windows, iOS and the web. There is no Android app, and Mem's help centre says it has no timeline to share for one, so an Android phone means the web app.
English is the only officially supported language across Voice Mode, Deep Search and Chat. Mem says other languages may work but are not supported, so treat non-English results as something to test before you rely on them.