You should learn most important principles of a chess game. That will give you a solid understanding of a chess game and you will know what to do in every position. Then, you should optimize your calculation so you don't miss tactics in your games. I can help you with all of this. Message me if you are interested.
beginner questions about chess

Start with endgames, then openings, then middle games.
Look up endgame simulator and chess tv videos or something is a good place to start.
Go through as many of those as you can, if you get stuck look up a video on that specific endgame for an explanation and do each one until you can do it 5 times in a row confidently.
For now, only work on up to 4 specific openings.
Generally, only move each piece once until you've connected your rooks and have castled to safety.
Try to keep the pawns in front of your castled king where they are until the later parts of the game or unless you have to. Avoid back rank checkmates and look for opportunities to do back rank checkmates.
When you're in the middle game, try to plan out your paths with your knights. Try to find the fastest, safe routes for them to take. Keep them off the edge of the board so they have more options unless it's the only safe way.
Learn about good and bad bishops.
Learn about when to simplify and when to avoid trades. Learn about tempo and zugzwang. Coordinate multiple pieces to attack specific spots. Find outposts for your knights that keep them close to the action. Learn how to define open and closed positions and what pieces play best in each. Keep your rooks back until the endgame. Take control and keep control of open and semi open files. Line up your minor pieces to attack your enemy queen through pieces that will be removed. Take control of the squares your opponents' pieces can move to so they can't move. Work on tactics and puzzles.
If you really really work on all of that, you'll be up 500 points in a year easy and up to 1000 in no time
@1
"What tactics/strategies are best to use in 1 minute chess games? (for a beginner with 700 ELO rating)" ++ None at all. Do not play 1|0 bullet. Play 15|10 rapid. One 15|10 rapid games helps you more than 5 blitz games 3|2 or 25 bullet games 1|0. The reason is that blitz or bullet are not worth to analyse and analysis of lost games is the key to improve.
"What is the most effective way to increase my 700 ELO?" ++ Always check your intended move is no blunder before you play it. Hang no pieces, hang no pawns. Sit on your hands.
"practising hard is not as effective as practicing smart. Any tips?"
++ The essence is to play and to analyse your lost games. Learn from your mistakes so as not to repeat them and to exploit similar future mistakes by your opponents.
@3
"Start with endgames, then openings, then middle games."
++ First endgames, then middle games, then openings.

Answers to some of your questions are addressed here...
Improving Your Chess - Resources for Beginners and Beyond...
https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond

What is the best chess website for free chess puzzles?
What are the differences between the most popular online chess websites where one can play games?
What tactics/strategies are best to use in 1 minute chess games? (for a beginner with 700 ELO rating)
What is the most effective way to increase my 700 ELO? (practising hard is not as effective as practicing smart. Any tips?)
Chesstempo.com for puzzles.
The 2 most popular now are chess.com and LiChess.org. Outside of the interface differences, the main difference is that some features are hidden behind a paywall on chess.com. Everything is free on LiChess.
Regarding your 2 questions about bullet chess: if your goal is to improve at Chess, do not play bullet.

What is the best chess website for free chess puzzles?
What are the differences between the most popular online chess websites where one can play games?
What tactics/strategies are best to use in 1 minute chess games? (for a beginner with 700 ELO rating)
What is the most effective way to increase my 700 ELO? (practising hard is not as effective as practicing smart. Any tips?)
Lichess for puzzles. Chess.com and lichess are the most popular sites, I like chess.com but it's just a matter of preference tbh. Don't play 1 minute if you're looking to improve. 15+10 much better.
What is the best chess website for free chess puzzles?
What are the differences between the most popular online chess websites where one can play games?
What tactics/strategies are best to use in 1 minute chess games? (for a beginner with 700 ELO rating)
What is the most effective way to increase my 700 ELO? (practising hard is not as effective as practicing smart. Any tips?)