There is a great site called repeater builder. The thing about repeaters is performance. Hams have had cross band UHF to VHF repeaters that are really simple for years. You pick two frequencies that are not harmonically related so interference is minimised connect the audio out to audio in, a little bit of switching, usually a line that goes high when one radio’s squelch opens that switches the other unit’s transmit on. If you (for maths example only) transmitted on 150MHz and received on 450MHz then the third harmonic of 150 would wreck it as the vhf radio would have enough output at 450 to make the receiver think a valid signal was coming in!
In band repeaters, on licenced frequencies means filters. Multi cavity devices that let through one frequency but block everything else, while at the same time allowing the other frequency to come in but blocking the one going out. These usually allow a single antenna but others require two antennas. They are needed because of a phenomena called desensitisation. If a receiver can hear somebody a fair way away on frequency 1, as soon as the transmitter on the repeater fires up it partially blocks that distant station, making the system deaf. The filters try to reduce this. The closer the receive and transmit are to each other, in frequency the better filters have to be. They need tuning and that is not simple unless you have an analyser with a tracking generator. If you have access to the kit, they’re not hard to tune. Some folk try it with less complex gear and it’s possible with skill and experience. I found an old repeater in my store I tuned years ago, and put the cavity on test. It’s performance was pretty poor. It was never very good on range, put down to location, but the problem was receive loss. The filtering was actually ok but the receive losses were quite bad.
That’s a quick trip to get you started.