I can't remember the first client I used, but it allowed me to check Jack's following and followers list, and had a "follow all" button. I did that with the ten or so biggest accounts, and that's how I got to follow 1700+ people. It is commonly said that once you follow at least 1000 accounts the TL gets interesting and engaging.
You can also do exactly the same with relays. Most clients if not all will let you see the relays someone else is using, and you can just add them. I would try not go over the top, I was using 40+ relays and it was awful. At 13-14 I think I'm in a good spot now.
On Android, Amethyst is hands down the best. On iOS, it's a mixed thing and I keep switching between Primal and Damus. On desktop, Primal and Iris.
And yes, you can log in "as" someone else with their npub (public key). In that case, you can see their TL and so on, but because you don't know their nsec (secret key) you can't post or do anything that requires signing. I think that's really cool actually.
#asknostr