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.