What would I be getting if I bought the player's handbook digitally?

Just curious because I don't have a lot of money, and I already plan to buy Monsters of the Multiverse, and a subscription to share content, so I want to know if it's worth it.

I've never looked at the freely available portion of the rules but here's a summary after a search that sounds right on some of the big things I have gathered in discussions here on DDB. It's entirely possible something more may have been added to the free rules since this summary, but in general you're getting substantially more player options. Whether or not this is important to you only you know. From an actual key game rules standpoint I don't have any familiarity with anything that might be missing from the free set.

• Classes/Subclasses: The paid PHB contains 48 total subclasses (4 per class), while the free version restricts players to one core subclass per class (for example only Thief Rogue, Life Cleric).
• Species & Backgrounds: The free rules limit options to 4 species and 4 backgrounds, missing most of the new origin options found in the full PHB.
• Feats & Spells: The free rules include a limited selection of 12 feats and about 320 spells, whereas the full PHB boasts all 75+ feats and a larger, revised spell list.

If you don't already own the Monster Manual I am a bit surprised you would be considering the Monsters of the Multiverse book, but hopefully you have already researched and understand the difference between the two.

Quote from spamandtuna >>

Quote from Allurian219 >>

If you don't already own the Monster Manual I am a bit surprised you would be considering the Monsters of the Multiverse book, but hopefully you have already researched and understand the difference between the two.

 If the goal is to get more character options, MOTM is definitely the better choice. It has more species than any other book. (The Monster Manual has no character options at all.)

 Very true. I would think most people would want the additional subclasses, species, and other player options from the Player's Handbook before trying to get the extra species from MOTM, but everyone has their own priorities. 

I think you mistyped - in the quoted text, you spoke of surprise that they were considering getting MotM before MM. Which, depending on what you want, is a viable option.

Quote from One1romanc3r >>

Just curious because I don't have a lot of money, and I already plan to buy Monsters of the Multiverse, and a subscription to share content, so I want to know if it's worth it.

 What is it that you actually want? Generally speaking, I'd recommend the PHB over the MotM. It has the full core rules, it may not have as many species options (ten, I think, versus thirty three in MotM), but it massively expands your subclass choice over the free options (thirty six new ones), a lot more spells, feats and so forth. MotM just has the additional species (some of which are in the PHB, now) and a bunch of new monsters to fight.

However, if all the monsters you want are either free or in MotM, and all the other options you want are free or in MotM, then MotM would make sense.

If you let us know what you're after, we can give you advice on what best suits you. Otherwise, we're just shooting in the dark hoping that we say something useful to you. That you're keen on getting the MotM but not sure about the PHB suggests that your needs aren't in line with the average player.

Quote from Linklite >>

I think you mistyped - in the quoted text, you spoke of surprise that they were considering getting MotM before MM. Which, depending on what you want, is a viable option.

 I didn't mistype, like your later post I'm not clear what the OP is really looking for or how much they know about DnD product contents. Since they are asking what they get from buying the Player's Handbook it seems possible they are new to the game and not fully familiar with the niche various products fill, but that could certainly be wrong.

The species in MotM might be the most important thing to someone, that's true, although it seems out of the ordinary. Someone also might really like the MotM monsters - I personally find them less useful than what you find in the Monster Manual, but everyone has their own preferences. My point was that I think usually people would buy the Player's Handbook first for a large number of player options, then the Monster Manual for the classic adversaries of high fantasy, then maybe later the Monsters of the Multiverse book to give more options once they had the core in place, hence why I said I hoped they understood the difference between the MM and MotM.

It depends really what you want, and whether you're talking 5e or 5.5e (the new PHB)

• Free gives you 1 subclass for each class, a couple of backgrounds and good chunk of feats.
• PHB expands the subclasses, backgrounds, feats etc significantly, to give the "base options" that people generally expect.
• MotM gives 'unusual' species, which while interesting, are definitely above and beyond.  However, they're 2014 designed, and there may be a 2024 update like many of the other ones.
• Forge of the Artificer gives the updated Artificer class (Tasha's and Rising from the Last War each had a version from 2014)

Another question for the OP is whether they already have the PHB in its physical form. If they do, then getting the digital version gives them the ability to use the content on here. 
If they haven’t, then I’d recommend getting the PHB over MotM: subclasses are, generally, more impactful on how a character functions than species. 
The next question is whether they just want to play a character or whether they want to be a DM. If a DM, then the DM’s Guide and the Monster Manual are probably better choices than MotM. If not, then MotM is a very reasonable choice, particularly if having access to those species appeals.
Do note that, of the 33 species in MotM, five have been updated: the Aasimar, Goliath and Orc are in the 2024 PHB -and the Changeling and Shifter are in the new Eberron book. That still leaves you with 28 species that are up-to-date. 
Although MotM was written before the 2024 PHB, the species generally work fine with the new rules (there’s a couple which need a bit of tweaking to work).