If your partner opens 1 Spade in 1st or 2nd seat and you have exactly 3 spades, 4 hearts, 3 diamonds, and 3 clubs, you bid a 3 card minor for 2/1. Some players will bid 3NT if they have 13-15 points with 3 spades and a balanced hand. You need 5 hearts for a 2/1 bid to 2H. With 16 or more points, bid a minor suit at the 2-level, 2/1 game force.
2/1 does not replace partnership conventions such as Jacoby 2NT. With a 4-card fit for opener's major suit, Jacoby 2NT would be preferred by many players who have that convention available.
2/1 is a NATURAL system with the priority to show shape. Bid your longest natural suit first. With two 4-card suits, you may choose the 2/1 suit if there is an option.
Do not bypass a 5-card spade suit to bid an artificial 2/1 suit as a GF. That would contort the shape description of the hand. 4th Suit Forcing (FSF) is an available Game Forcing option.
2/1. With nothing better to do, bid your lowest ranking 4-card minor (on RARE occasion a 3-card minor) to create the GF then bid shape naturally to explore for a contract.
Once you have responded to a 1S or 1H opening bid, the conversation is underway and the information exchanged up to this point does not change. Interference from LHO means opener is no longer forced to bid. Competitive bidding takes over from here. Any further competitive action by responder is limited to an invitational maximum of 11-12 points and information previously exchanged. With nothing better to do, sometimes it's best to defend, try to defeat the contract, and get on to the next hand.