Lemmy

• https://browse.feddit.de (search communities across all instances, seems to work better than the search built-in to Lemmy instances)
• https://join-lemmy.org/instances
• https://lemmyverse.net

Lemmy communities (think subreddits) are given in this format: !communityName@instanceName.
Since Lemmy is on the Fediverse, you can follow Lemmy communities (think subreddits) from Mastodon, just replace the ! by @, and you should find the account name.

Lemmy Android apps:

• https://join-lemmy.org/apps
• Jerboa, Android app made by Lemmy's developers [I could not figure out how to switch to another instance without logging in first]
• lemmur is listed on F-Droid, but on its Github page the app clearly states that it's no longer maintained.