How'd LA, Boston build their teams? Like this ...

A year ago, the World Series pitted the best team in the American League, the Houston Astros, against the Los Angeles Dodgers. The cast has changed a bit this time around, but it's once again the Dodgers facing the best the AL has to offer -- this time, the Boston Red Sox.
For the second year in a row, the AL team that had the highest cumulative WAR among the players on its ALCS roster is playing for a World Series ring. The Red Sox's 57.5 WAR for their ALCS roster ranked just ahead of the Astros (56.5), the team they just beat. With Drew Pomeranz replacing Brandon Workman on the Red Sox's World Series roster, Boston's WAR drops to 56.1.
In 2017, the Dodgers ranked second among NL playoff teams in WAR. This time around, they were easily No. 1, at 54.7, with the just-dispatched Brewers second at 45.0.

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In many ways, these two World Series rosters mirror each other, not only in how they were constructed, but in where the value has come from. Both World Series participants have used trades more than any other avenue to build their potential championship squads, with some of the smaller deals being as, if not more, impactful than any blockbusters.
Both teams have a fair amount of homegrown talent, with the Dodgers having the edge there. The Red Sox have played more actively on the free-agent market, while the Dodgers have had some under-the-radar signings pay huge dividends.
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Here's a closer look at how these organizations used the Draft, international signings, the free-agent market and trades to build their teams, with each category broken down by the number of players on the team's postseason roster and the value each player had over the course of the season, as calculated by Baseball-Reference's Wins Above Replacement measurement.
HOMEGROWN (Draft + international)
Red Sox: 8 players, 20.7 WAR
Dodgers: 10 players, 21.3 WAR
DRAFT
Red Sox: 6 players, 16.9 WAR
Matt Barnes (1.1), Andrew Benintendi (3.9), Mookie Betts (10.9), Jackie Bradley Jr. (2.1), Blake Swihart (-0.3), Christian Vázquez (-0.8)
The 2011 Draft has proven to be very fruitful for the Red Sox, even if it's taken some time. Barnes was the team's first pick at No. 19 overall, and while it took him some time to find his niche, he's become an integral part of the bullpen. Seven picks later, Swihart was taken from the New Mexico high school ranks, while the organization got Bradley Jr. coming off of a subpar junior season at the University of South Carolina at No. 40. Of course the steal of the Draft was getting Betts, the MVP candidate, in the fifth round. For most of the year, he, Bradley Jr., and Andrew Benintendi (No. 7 overall pick in 2015) have made up an all-homegrown outfield.
Dodgers: 4 players, 14.0 WAR
Cody Bellinger (4.2), Walker Buehler (3.5), Clayton Kershaw (4.0), Joc Pederson (2.3)
While the Dodgers tend to be known for big trades and big free-agent acquisitions, they had as many homegrown players on their postseason roster as any team in this year's playoffs. They didn't miss when given a top 10 pick (Kershaw, No. 7 in 2006) or when they were picking toward the end of the first round (Buehler, No. 24, 2015), but they also did well in identifying talent in later rounds and going over slot to sign them, nabbing Bellinger in the fourth round of the 2013 Draft and Pederson in the 11th round back in 2010.
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INTERNATIONAL
Red Sox: 2 players, 3.8 WAR
Xander Bogaerts (3.8), Rafael Devers (0.0)
Devers had an uneven regular season but is coming off of a big ALCS (.385 with a homer and six RBIs) while Bogaerts has settled in at shortstop quite nicely.
Dodgers: 6 players, 7.3 WAR
Julio Urías (0.3), Pedro Báez (0.7), Kenley Jansen (0.6), Kenta Maeda (0.4), Yasiel Puig (2.7), Hyun Jin Ryu (2.6)
No team has more international presence than Los Angeles, and the Dodgers have paid handsomely to do it, giving seven figures and then some for Maeda, Puig and Ryu.
FREE AGENTS
Red Sox: 4 players, 13.2 WAR
Dodgers: 2 players, 8.7 WAR 
Red Sox:Ryan Brasier (1.5), J.D. Martinez (6.4), Mitch Moreland (0.9), David Price (4.4)
The Red Sox are looking into manipulating their defensive alignment to get Martinez, whose five-year contract this offseason drew criticism from some, into the lineup in the NL park. Price is coming off perhaps the best postseason outing of his career in Game 5 of the ALCS. But with those big names, it might be Brasier who is the most interesting free-agent signing, and a Minor League one at that. Beginning the year in the Minors after pitching in Japan in 2017, Brasier has become indispensable out of the pen, pitching in seven postseason games and posting seven shutout innings.
Dodgers:Max Muncy (4.2), Justin Turner (4.5)
They may have been small and quiet at the time -- both were Minor League deals -- but it's hard to imagine this Dodgers team being where they are without either of these free-agent signings. Turner's impact has been long-standing, as he's been a top 10 MVP vote-getter twice and an All-Star since signing in 2014. The '17 NLCS MVP has helped the Dodgers reach the postseason every year since he came to Los Angeles. Muncy started the year as a utility type, then settled in at first base, with his 35 homers tying for fifth in the NL this season.
TRADES
Red Sox: 13 players, 22.2 WAR
Dodgers: 13 players, 24.7 WAR
Red Sox:Nathan Eovaldi (1.5), Heath Hembree (1.7), Brock Holt (1.3), Joe Kelly (0.5), Craig Kimbrel (2.3), Ian Kinsler (2.4), Sandy León (-0.5), Eduardo Núñez (-1.1), Steve Pearce (1.4), Drew Pomeranz (-0.5), Rick Porcello (3.3), Eduardo Rodriguez (3.0), Chris Sale (6.9)
Boston's postseason pitching staff has largely been built via trades. Five wins and five saves (all from Kimbrel) came from trade acquisitions this postseason. Kinsler was their big Deadline deal, and he's helped solidify second base with Dustin Pedroia hurt, though getting Eovaldi from the Rays may have made more of an impact, as he's been one of the most consistent arms on the staff. They've also found value in smaller deals, like the one in June for Pearce, who has become the regular first baseman on this squad.
Dodgers: Scott Alexander (0.3), Austin Barnes (0.5), Brian Dozier (1.0), Ryan Madson (-0.6), Dylan Floro (1.8), David Freese (2.1), Yasmani Grandal (3.3), Kiké Hernández (2.8), Rich Hill (1.3), Matt Kemp (1.1), Manny Machado (5.7), Alex Wood (1.3), Chris Taylor (4.1)
As little credit as the Dodgers get for their homegrown-ness, the assessment that they can, and do, go out and make trades to help get them this far is completely on point. Sometimes it's smaller trades, like for Taylor or Hill, that have a larger impact than anticipated. But for two years in a row, they've gone out and made big deals at the Deadline for the postseason. Last year, it was Yu Darvish; this year, obviously, it was Machado, who is coming off a contentious NLCS, but one in which he hit .296 (8-for-27) with a double, a homer and three RBIs.