I guess the main thing that really confused me is that I remember people stating that if they came into X number of dollars (Like $100k) they would just dump it all into VTSAX and forget about it. Apparently that's not actually possible in lump sum.
It is possible in a taxable brokerage account, as terran mentioned. No limits there.
Right.
It's possible you're confusing Accounts (401(k), Roth IRA, traditional IRA, taxable/brokerage, etc) with investments (VTSAX, VTIAX, etc). One puts money in an account, and buys investments with the money in that account. So the account and the investment are separate decisions. While some accounts have annual contribution limits, others (taxable/brokerage) don't, so you can put as much money as you want in a taxable/brokerage account and you can invest that money in the same investments as other accounts (for the most part).