Just to be clear on the league vs inter-league...
U15 and U17 MLS Academies are in the "Pro Player Development" League. These are the regional groupings for MLS teams. Then you have the non-MLS teams that are the official MLS Next league. When an MLS team plays a non-MLS team, it is an inter-league game. That is why non-MLS teams only play MLS teams once, if at all... whereas the MLS teams will play each other twice (and one opponent three times), with about half of their games coinciding with the first team. For example, LAG and LAFC play this Sunday... as does the first team. RSL played in Colorado when their first team played as well.
LAFC and LAG are not in the same league as TFA, MSA, Ventura, etc. They can qualify for the MLS Next Playoffs and if not, the MLS Pro Player Showcase coinciding with the Playoffs. They will not play in the MLS Next Showcase (as they are not technically in the MLS Next league).
MLS academies will play 15 games in their Pro Player league, 3 games in the playoffs (or their showcase), 3 games in the Winter Event, 3 in GA Cup (if they qualify) and 16 inter-league games.
The west group of the Pro Player Development League includes LAG, LAFC, RSL, San Jose, Colorado, Vancouver, Seattle, and Portland. I believe there are three divisions of the Pro Player Development League.
In the Pro Player Development League, an MLS team plays everyone home and away, then plays their closest rival one additional (a third) time.