What's the best way to grow the steam library?

Money

Steamgifts. Stay here for 10 years and win 400 games in the meantime.
pray to Satan for better rng

View attached image.

This would be the easiest, but chances like this don't come often:

https://venturebeat.com/games/you-could-own-every-steam-game-if-youre-the-grand-prize-winner-of-the-holiday-sale/

Indie bundles from Humble Bundle, and Fanatical.
For about $100 you can easily collect several dozen rather excellent games this way.
Also waiting for steam winter and summer sales can also save you money on games you want but dont seem likely to be in a bundle.
Check out steamdb.info to get more info on specific games and their history of going on sale and such.

https://gg.deals/

If you don't mind games that give no +1 to your library get the SteamDB browser extension and you can add around 9,000 Steam games to your library (Steam limits this to 50 per hour so you'll need some persistence).

Obviously sales, bundles, cheap keys from grey marketplaces.

Have an OnlyFans

The best way to grow your steam library is to buy a game you like to play. And then play the game until you don't have fun anymore. Then buy another game you want to play. It is the cheapest path to gaming.

Buying a cheap game you never play is expensive.

Inheritance

What's the point of growing a Steam library? Why not just buy games you like and play that until you get tired of it?

Don't.

Just curious: what for?

make sure to check deals like 90% off which come around a lot for different games

I think almost all the answers are partially right (except only fans-lol). I bought a lot of bundles years ago, in the past you could get several free games every month, won many games here, spent a lot of money on steam when I could afford that and was gifted many games. All that contributed to the library I have today.
There's no easy way and you surely will have to spend money.

buy games you like

go to twitch and watch a channel called hit squad god father they change your points into games

If you're talking about +1's (which I assume you are and if not someone left a guide for getting the free games on steam somewhere above)
you sort of explained it yourself, going on either official bundle sites (Humble, Fanatical, Indiegala, even DIG in some cases although DIG rarely if ever has +1 games) or grey market which you can probably find games for cheap

Although i'd not recommend just growing your steam library for the sake of it, just going around the sites above and frequenting steam gifts will eventually grow it passively with games you'll actually enjoy instead of random junk you'll never touch, and if you're really just after the xp which +1 grants from levelling the badge, just buy steam cards at that point, it'll save you a lot of money just to raise the steam level than growing your steam library with +1's.

Buy bundles, keep what you want, trade the rest for games from bundles you missed, and buy bundled games from resellers. You can use barter.vg to find good games that many people are trading and then find the cheapest prices with deals.gg