Overwatch 2

Ways of amassing experience are as follows:
3.4 XP per second that you are in a match
Gold Medal - 150 XP (Only get XP for highest medal earned)
Silver Medal - 100 XP (Only get XP for highest medal earned)
Bronze Medal - 50 XP (Only get XP for highest medal earned)
Finishing Game - 250 XP
Winning Game - 500 XP
Playing another game after finishing a match (continuos matches) - 200 XP
First Win of the Day - 1500 XP
Backfill XP (Joining a match in progress) - 400 XP

+20% Bonus XP of all XP earned during a match when being in a group with at least one other person. This is crucial to gaining experience the fastest way possible.

Level -> XP Needed to Level Up To Next Level
1 -> 1500
2 -> 3000
3 -> 4500
4 -> 6000
5 -> 7500
6 -> 8500
7 -> 9500
8 -> 10500
9 -> 11500
10 -> 12500
11 -> 13500
12 -> 14500
13 -> 15500
14 -> 16500
15 -> 17500
16 -> 18500
17 -> 19000
18 -> 19500
19 -> 20000
20 -> 20500
21 -> 21000
22 -> 21500
23-50 -> 22000

Information from this guide ascertained from personal in-game experience, and xp per level information found on various locations on the internet that have been confirmed factual.

Edit: Be aware that there is a -75% experience reduction when you leave 3 games out of 20. Just information to add, as noted in the comments, as it pertains to experience.

Achievement has now changed since the previous solution.

You have to hit Level 50 on the Battle Pass. Well, at the time of writing, it is 45. Seems as if the achievement is tied to whatever level the unlockable character is. Although, this may just be a glitch for Season 4.

Each level requires 10,000 XP. You can get 9,000 XP from your daily quests alone. Completion of a game will give you several hundred XP too. With about 45 minutes of gameplay per day, you can unlock this achievement in 50 days.

If you look at your 'Challenges' tab from the menu, this will list all challenges that award XP. There is a cap on daily XP (3 challenges), but you can complete everything under Weekly, Event, Season, etc. I only aimed for daily XP challenges as 1 level per day is enough.

Remember, you only have until the end of a season to earn this. Once the season ends, you have to start all over again. So, do your best to get in 1 season!

Oh, and daily achievements reset ever 19 hours. However, they ONLY reset if you log in to the game. So, if the 19 hour timer expires when you are NOT playing, a new one won't start until you are in the game again.

Game is crossplay. You can play your challenges on ANY device (I played mine on Switch), but you have to be on the Xbox when you hit the target level, or there is a risk it won't unlock.