Thanks for the A2A.
There are many mutual fund and investment apps out there and yes if they are registered with Association of Mutual Funds of India (AMFI) or Securities and Exchange Board of India (SEBI), they are legitimate.
However every Mutual Fund come in two plans – Direct Plan and Regular Plan. A direct plan (one with the words Direct Plan in its full scheme name) gives you additional return of upto 1.5% because it does not pay out indirect commission to the broker. With direct plans, you get upto 40% more in long term portfolio value, by avoiding commissions and brokers.
Go for an app or platform that aggregates direct plans (for a small subscription) instead of huge indirect fees. Dont assume that ‘free’ means really free!